The first example is Jefferson Airplane’s “Surrealistic Pillow” album, where Garcia was credited as “musical & spiritual adviser.” He stayed with the Airplane through many of the album sessions, playing on several songs, helping with arrangements, and effectively acting as unofficial co-producer for the album. This is especially surprising since the Airplane had already made a successful album, while Garcia and the Dead only had one failed single to their name.
More details here: http://deadessays.blogspot.com/2010/12/jerry-garcia-surrealistic-pillow.html
The New Riders of the Purple Sage, of course, would not exist without Garcia, who was the co-founder of the band. I’m sure John Dawson would have formed another group to do his songs, and it may even have included a pedal-steel player; but it would probably have sounded rather different, and it certainly would not have had the constant initial exposure to audiences the New Riders had, opening for the Dead for two years straight.
David Crosby’s first solo album, “If I Could Only Remember My Name,” would have been a bit different without Garcia’s contributions. Crosby later said, “Jerry Garcia is responsible for that record a very great deal. He was there night after night after night…thinking, listening, talking - you know, acting as a friend, saying ‘Hmmm, man, what if you, how did you, why don't you try a little more, and....’ And he would play. He played on a lot of stuff.”  "Jerry must have come around to Wally Heider's every night for a long time, and he was so helpful, so bright, so inventive... If I had to elect one musician to represent musicians in the Galactic Congress of Beings, it would have been him." 
Garcia was also heavily involved with “Blows Against the Empire” and later Paul Kantner/Grace Slick albums, and apparently had quite a bit of creative input. “Blows,” Crosby’s album, and “American Beauty” were being made at Wally Heider’s at the same time, so as Kantner said, “We would just wander into each other's studios, listen for a while, and say, 'I could add something to that.’”  (Note that it was Garcia who contributed to the others’ albums, not the other way around – perhaps because the Dead’s music was more simple and insular at that point.)
Kantner recalled, “Jerry was doing a lot of pedal steel for people around that time, experimenting, and so we let him be on it; he was overjoyed. So he went in and just experimented with sounds, seeing what kind of sounds he could get out of it, running it through various pedals and echoes and delays. We gave him a free hand, which made him happy. Before that he’d pretty much just been doing country licks on the steel, and this gave him the opportunity to get a little weirder, which he always appreciated.” 
Garcia spent a lot of time at the PERRO sessions at Wally Heider’s, most of which wasn’t released, but fed into later Crosby or Kantner albums. (For instance, ‘Mountain Song,’ which Garcia co-wrote and originally sang, was released in a later Kantner recording in 1983.)
Howard Wales’ “Hooteroll” album would not have been recorded without Garcia. In fact, it’s hard to say when or if Wales would ever have pursued a solo recording career, without Garcia’s initial support – Wales would only release one more album in the ‘70s, preferring to stay in the shadows. Garcia would later say of his one east-coast tour with Wales supporting the album in January ’72, “I didn’t really go on the road that time to play. The thing was really misrepresented. I just wanted to get Howard out playing, and the band had a really nice thing going which really didn’t have much to do with me. I was just there fucking around.” 
Merl Saunders’ early albums, “Heavy Turbulence” and “Fire Up,” would have been quite different without Garcia’s involvement. Saunders recalled, “I had an album to do over at Fantasy Records. I said to Jerry, ‘You helped me develop these tunes, you might as well come and do them.’” 
Not only was the material much of the same stuff they were performing live, Garcia also gave Saunders some needed moral support when making the albums. As Saunders said, “Jerry always encouraged me to write material for the group. [The record label] Fantasy was saying stuff like, 'The music is great, but these words about ecology and whatnot, you gotta tone 'em down.' I said, 'I like the words.' And Jerry's attitude was, 'Yeah, fuck 'em! Do what you want, Merl!' He was really my inspiration to do things the way I wanted to do them. I would maybe be leaning toward giving in to the record company and he'd say, 'Merl, you wrote these songs from the heart, so fuck 'em. It's your music, man. Put it out the way you want to.' And I needed to hear that.” 
As well, Saunders said, “He also got me singing. We’d come off a tour and he’d say, ‘Man, you gotta help me out singing.’ I didn’t think I could sing, but I figured if Jerry could sing, I could sing. He’d say, ‘You hear how bad I can sound; you don’t sound any worse than I do.’ I said, ‘I guess I don’t.’ We used to laugh about it, but that’s how I started singing.” 
The Garcia/Saunders band also played on Tom Fogerty’s second solo album “Excalibur” in 1972. (At the time, Fogerty was more or less a member of the band.)
Much later, Garcia also helped Saunders record his “Blues From the Rainforest” album in 1989.
Old And In The Way, technically speaking, probably does not belong in this post. Garcia started the band informally as a way to play bluegrass in public with some friends – David Grisman recalled, “We just got together one time in Jerry’s living room and started playing bluegrass and Jerry said, ‘Wow, we ought to go play some gigs.’ Me and Pete [Rowan] probably needed the bread.” 
It was a blip in the careers of Garcia, Grisman, Rowan, and Clements. Nonetheless, their album became a bestseller and extremely influential in the bluegrass field, exposing many new listeners to the music. (It’s even arguable that Garcia is a more important figure within country and bluegrass music than in rock music.)
Garcia was asked in 1991, “Is Old & In The Way still the biggest-selling bluegrass album?”
“That’s what I’ve heard. I don’t know where those statistics come from, but I think that’s tremendously flattering if it is. Although kind of unfortunate. I mean, what about those guys who spent their lives playing this music? They were certainly great at it.” 
The Round Records label would not have existed without Garcia. While a couple of the Round albums would no doubt have been made on other labels, it is pretty safe to say that without Garcia steering the label, there would have been no Keith & Donna album, no Seastones album, and no Diga Rhythm Band album – all of which he played on and helped produce. (Garcia was frequently involved with Mickey Hart’s solo efforts through the early ‘70s, as you might expect, but I think that’s a subject that falls outside this post.)
Robert Hunter would also most likely not have recorded his first two albums, “Tales of the Great Rum Runners” and “Tiger Rose,” which Garcia produced.
These are familiar Dead-related names, but in Round Records’ brief history, Garcia also got a chance to step into the bluegrass field, with the Good Old Boys’ “Pistol Packin’ Mama” album. Garcia gathered together a group of some of his favorite bluegrass musicians to make the record, which he produced. Unfortunately, given the troubles of Round Records, it quickly went out print and sank into oblivion. If the label had continued, though, it’s possible Garcia would have continued to do sessions like this for other musicians he liked.
It’s also worth mentioning that Garcia had a direct financial stake in these albums – when they failed, he lost money. Round Records was basically formed to protect the rest of the Grateful Dead, as a home for “financially dubious solo projects,” and Garcia and Ron Rakow were the sole owners (and apparently sole overseers) of the label. As the JGMF blog notes, “It really speaks to how committed Jerry was to the other GD members, the Family, etc. He was not only subsidizing their solo projects (did anyone really think that Diga or Seastones or Keith and Donna would sell?), but also taking on disproportionate risk… Jerry was pretty much carrying everyone and everything.” 
[These posts also provide more detail on Round Records and the various album projects:]
Sometimes Garcia also helped out on other musicians’ projects that fizzled out.
Garcia & Grisman played on a demo session for a prospective Peter Rowan solo album, but the album was never made and the tracks weren’t released til 1980.
Garcia had played on studio sessions with the Rowan Brothers back in 1971 or so – he was excited enough about that band to rave about them in a Rolling Stone interview, which backfired when his comment that “they could be like the Beatles” was used to promote their album, creating many disappointed listeners. (Audiences were also unimpressed when they opened for the Dead.)
One session Garcia did that wasn’t heard for many years was with Bill Cutler in 1975 – which wasn’t released until 2008. Cutler had encountered Garcia at Matt Kelly’s studio sessions: “In the course of seeing him around, he heard a bunch of my songs and said, ‘If you ever make an album of your stuff, I’d be happy to help out.’ Which was a pretty exciting offer, of course! In fact, I’m not sure I even believed it at first… Jerry heard that I was going in to record and told me he wanted to play on the tracks. Because the Dead weren’t touring then, he actually had some time and he came by, rehearsed a little with us, and we recorded about half an album with him. He seemed really comfortable in the group… The plan was to cut a bunch more tracks at some point, but what happened is the Grateful Dead went back out onto the road and then they got so busy that there was just never an opportunity to do more, so I literally put the tapes in a closet and they sat there for many, many years.” In 1993 Cutler bumped into Garcia again: “He asked me about what had happened to the tracks we’d recorded at Heider’s all those years ago. He said, ‘We should finish that album!’” 
There may be more ‘lost’ sessions with other musicians we don’t know about if they haven’t been released
Garcia also encouraged and frequently participated in Ned Lagin’s music. Aside from Lagin’s appearances at Dead shows, Garcia also went out to play with Lagin at a couple “Seastones” shows (most notably 6/6/75), though they parted ways after that. In this case, Phil Lesh participated more than Garcia in what there was of Lagin’s solo career. Lesh said in 1980: “Ned went his own way after that, although we still communicate… We essentially did a benefit for him and got him a computer and a synthesizer.” 
Garcia also made a dramatic reentry into David Grisman’s music in the 1990s. Grisman reconnected with Garcia after years of not talking, and in 1990 Garcia ensured that Grisman received a Rex Foundation cash grant (which would help fund releases on Grisman’s label Acoustic Disc). Grisman recalled, “Later I found out that Jerry was responsible and I called him to thank him… Jerry came over to my house one day, checked out my home studio and asked me, ‘How about putting out some more Old and in the Way tapes?’ I said, ‘Frankly, Jerry, I’d rather see us put out something new. We can put out the old tapes when we’re in wheelchairs.’” (Garcia would idly muse about getting Old & In The Way back together, but it never happened.)
Garcia returned to Grisman’s house with an acoustic guitar: “He just walked right in and said, ‘We should make a record and that would give us a reason to play.’ …When I told him about my small record company [Acoustic Disc], he said, ‘Great, so we can do it for you.’ When I asked, ‘When do you want to start?’ he said, ‘Now!’” 
Per deaddisc.com, “The Garcia and Grisman partnership also gave Grisman the finances needed to continue his Acoustic Disc record label. As Grisman noted in another interview, ‘Jerry kind of takes care of the profitable part.’”
In a smaller role, Garcia played pedal steel for various artists in numerous studio sessions from ’69 to ’73. This isn’t the place to list them (see http://deaddisc.com/GDFD_JGPerformer.htm ) but it’s notable that pretty much all these sessions were for musicians who were his friends & acquaintances. (That is, he wasn’t an in-demand session man for those outside his circle.) His contributions are generally limited to one or two songs, so he clearly wasn’t heavily involved in the recording process, just being called in for a solo or two. Sometimes his parts weren’t even released, so his role on some albums is unknown. Frequently he was an anonymous player, not listed in the credits. (For instance, Steve Stills omitted Garcia’s name on his albums. Also, on Lamb’s 1971 album “Cross Between,” he played on some tracks but wasn’t credited – instead there was a note on the back cover: “Special thanks to Jerry Garcia.”)
It’s also notable that the most famous notes he ever played were from one of his first guest sessions, CSNY’s ‘Teach Your Children.’ As such a well-known song, and one of the most recognized pedal-steel tracks ever, it can start bitter quarrels about Garcia on pedal-steel forums to this day. (“Mediocre” is the general opinion.)
John Dawson remembered that Crosby and Nash “came up and sang Jerry and me 'Teach Your Children,' and they asked if he'd play pedal steel on it. They were at Wally Heider's in San Francisco working on their second album, and so Jerry said, 'Sure, man, yeah. Absolutely.' So we went over there…and we went into Studio D, at the back end of the hallway. They set Garcia up in the studio, played him the tape two or three times, and then Crosby said, 'OK, let's roll some tape.' So they did a take, and then another, and then Garcia said, 'Let's roll it again,' and Crosby said, 'Nuh-uh. That's it. That's perfect. Get out of here.'” Garcia’s part was done in two takes. 
Garcia later said, “Nice tune. Nice note. I got one good note in on that tune! One good note makes it worthwhile! [Laughter]” But he added, “I really think the nicest thing I did during that period was on Crosby’s solo album… I particularly like the pedal steel on ‘Laughing.’ That was some of the prettiest and most successful of what I was trying to get at at that time.” 
Garcia was asked in 1983 about creating a definitive Garcia discography. “I’ve done so many sessions, I don’t keep track…I just don’t think of it in [completist] terms, because during the period of time when I was doing lots and lots of recording sessions, from about ’69 to ’73 or ’74, right around there, I did a lot of sessions, and the reason I did them was because I wanted to get more studio experience in, and because I liked the problem-solving mentality that you get into. People would call me in because they wanted what I could contribute to the session. That was my function. That was the reason I was there. And that was the way I perceived the work I was doing. In those moments I’m interested mostly in the music, not in myself. I don’t do those things for my own career! I do them for the music at hand and because at the time I really enjoyed doing sessions… There were so many things. We all got involved in all kinds of little side projects and odd little one-shot things… There have been some things I played on where I’m not credited at all… Some of those I just don’t remember. Sometimes they were very weird experiences, like, for those I didn’t know what records they were for or anything.” 
After the Round Records era, Garcia stopped playing on other people’s sessions for a long time. From 1987-1995 he would do occasional guest appearances again, mostly on the albums of friends or musicians he liked. I don’t have much to say about these (they could be a post of their own), but they show that even when the demands on his time were most intense, he would still go out of his way to spend time playing for the musicians in his circle, adapting himself to what they wanted on their tracks.
There is also a parallel in his jamming habits. While he was insatiable for opportunities to play with others, he would not always take the lead, but was just as happy to stay back and support others in the jam, sticking to rhythm backing and letting them direct the music. Garcia would sometimes say that he was happiest being a sideman – for instance, in bluegrass music: “I don’t really want to front a band. I would rather play in a bluegrass band as a banjo player, like in Old & In The Way, and let somebody else front it… I don’t think of myself as a lead singer. Gimme a couple tunes in a set, and I’m perfectly happy. And I love singing parts… I think of myself as an accompanist. That’s my field.” 
This could be another topic of its own.
There are many stories of Garcia encouraging other musicians:
Martin Fierro recalled, “Jerry encouraged me to bring in new tunes. Oh, man we were playing all sorts of stuff…He was always up for anything. I asked him one time, ‘Do you mind if I play to my heart’s content?’ and he said, ‘No, man, I want you to. It makes me play better when you give your all, Martin.’ You play with some guys and they don’t want you to play a lot and you can’t really express yourself, but it was never that way with Jerry. We were all on the same page and we were all equals.” 
Peter Rowan recalled, “Jerry had this quality of reciprocal enthusiasm and the ability to give off a kind of light. If you tickled his fancy, he would just come forth with so much loving energy that everyone would do better. When you played with Garcia, he could make you rise up to your full capacity. He could make you do that and I think it was reciprocal… I never experienced any ego when he played. He was against any discursive criticism of the moment. He could make every player feel that whatever part they had to contribute was part of the overall experience. He was generous.” 
Michael Lydon, who was then a Rolling Stone reporter hoping to become a musician, spoke to Garcia in 1969: “When I confessed to him my own musical hopes, his instant, smiling response was, ‘Yeah, man, do it! Whatever I am doing, you can do it too!’” 
Garcia’s support of others wasn’t just outside the Dead, though. Though he was reluctant to act as the “leader,” a lot of things that happened in the band were the result of his decisions. In particular, he frequently encouraged the other members to step up more and sing their own songs – whether it be Pigpen, Brent, Weir, or Donna. He was eager for the others to take on a bigger role in contributing material, and seems to have been supportive of whatever was brought in.
For instance, in 1974 he was already complaining, “To me, just because of default, I've fallen into the role of being the main writer in the band. And I'm not really a writer, I'm not really a composer… But these are roles, and since the band has needed them I've fallen into them, just like we all have. But it's been on me to be the guy who's developing the material. And frankly, I'm tired of my own writing, I'm bored with it. Since it's sort of an artificial situation, I'm not an inspired writer. It represents work.” 
This was an ominous foretelling of Garcia’s future decline as a songwriter. By the ‘80s Brent Mydland would be writing more songs for the Dead than Garcia was, to Garcia’s steady approval. Blair Jackson writes that “Garcia liked Brent and his songs, and went out of his way to encourage him to write more.” Garcia said after the “Built to Last” album, “It was Brent that had the good songs – I mean, more of ‘em… It’s nice to be able to show off what he can do on a lot of different levels. And his contribution to this record is really outstanding all over. Not just his tunes and vocals, but everything else – all the keyboard parts and just ideas and general stuff.” 
If Garcia hadn’t asked Robert Hunter to write songs for the Dead, it seems unlikely that Hunter would have become a songwriter: “I had written lyrics on and off since I was 17, but I fancied myself a serious writer, and rock & roll wasn’t exactly what I had planned for myself.”  “Without the GD I’d be just another guy who didn’t get a chance to develop what talent he had.” 
And of course, we would probably never have heard of Phil Lesh if Garcia hadn’t asked him to play bass for the band: “I know you’re a musician – you can pick up this instrument so easy.”
Lesh later mused, “If I had never taken this road, who knows what I’d be doing? It’s so impossible to even consider it… I don’t know. I probably would have left music forever. I already had.” 
2. Voyage box set liner notes
4. Jackson, Garcia p. 185
5. Jackson, Garcia p. 228
6. Greenfield, Dark Star p. 138
8. Troy, Captain Trips p. 160
9. Greenfield, Dark Star p. 152
10. Jones & Pickard, “Crazy Fingers: Jerry Garcia & The Banjo,” Relix 19, 1992
13. “Phil Lesh: Reddy Killowatt Speaks,” Comstock Lode 9
14. Jackson, Garcia p. 400
16. Hunt, “Jerry Garcia: Folk, Bluegrass & Beyond” pt II, Swing 51 #7, 1983
18. Jones & Pickard, “Crazy Fingers: Jerry Garcia & The Banjo,” Relix 19, 1992
20. Greenfield, Dark Star p. 154
21. Lydon, Flashbacks p. 96
23. Jackson, Garcia p. 383
24. Gans, Conversations with the Dead p. 23
26. Gans, Conversations p. 201