Donovan taught Lennon the guitar finger picking style he would use on Dear Prudence, Julia, Look at Me, etc. Donovan also added the lyric, sky of blue, sea of green to Yellow Submarine when Paul was struggling filling the gap.... Whoops I got so carried away with reading this post I forgot it wasn't a did you know but who played on...
Ron Wood and David Bowie both play on The Rolling Stones' It's Only Rock and Roll (hell. Wood actually wrote it)
Savoy Brown is the backing band on about half the tracks on Dana Gillespie's 1969 lp, 'Box of Surprises.'
Trevor Rabin (formerly of Yes, and primary writer of "Owner Of A Lonely Heart") plays the brief guitar solo (at ~3:00) on Michael Jackson's 1996 hit "They Don't Care About Us".
Rick Wakeman also played Mellotron on Bowie's "Space Oddity". According to my 1979 YES tour program, Wakeman states that was best thing he ever did...
Hal Blaine played drums on "Ventura Highway" by America. Notably, it's not in his usual style. And, of course, Hal played on hundreds if not thousands of other hits.
The doo-dee-doot-doos on Bowie's Sound and Vision are Mary Hopkin, who was Visconti's wife at the time. You can hear Iggy in the mix on What In The World, too. Bowie provides some uncredited background vocals on the title track of Reflektor by Arcade Fire.
Marc Bolan plays twin lead guitar on the track Ma-Ma-Ma Belle alongside Jeff Lynne and features on a number of takes from the April 1973 ELO session, such as "Dreaming of 4000". (from Wikipedia)
Merry Clayton, perhaps best known for her duet with Mick Jagger on "Gimme Shelter", also sang backup vocals on Lynyrd Skynyrd's "Sweet Home Alabama".
The Revolution's Wendy Melvoin and Lisa Coleman play on Neil Finn's 2001 solo album One Nil (aka One All). Wendy also co-wrote four songs with Finn, and Lisa one.
He is also on Roy Harper's 'Stormcock' album, under the alias S. Flavius Mercurius, for contractual reasons.
Whilst I agree that Neil's guitar is "one for the ages", Jimmy Page's guitar around a different Davy Jones vocal is of its time. The Manish Boys - "I Pity the Fool" 1965
The four members of 10cc played, arranged, engineered, and probably even bought the tea in, on Neil Sedaka's 'Solitaire' and 'Tra-La-La' albums in the early 70s..
Paul McCartney performs backing vocals on Godley & Creme's 'Get Well Soon' (album 'Freeze Frame'). He isn't mentioned on the sleeve, but, like the other contributors, represented by a small picture.
John Paul Jones wrote some very average string arrangements on Automatic For the People. At least four members of Split Enz play on the first Crowded House album.
