Pretty much any big-budget 70s-80s album produced by Quincy Jones has already-famous singers doing backing vocals on some tracks. Michael Jackson, Roberta Flack, James Ingram, Chaka Khan, to name a few. And that’s without mentioning instrumentalists.
It was quite incestuous in a way. And yet, to find out they were there, you pretty much had to scrutinise the personnel lists on the sleeve.
David Bowie on Queen’s “Under Pressure” beats the balls of anything mentioned so far.
Not actually backup singing in the strictest sense, but Stevie Wonder did the harmonica work on Chaka Khan’s “I Feel For You”, which was really the only memorable aspect of the song aside from the dude rapping about what he wanted to do to Chaka Khan.
Joe Jackson sang with William Shatner in 2004 on the song “Common People”. I have no idea who was backing up whom and I do not care to speculate on it.
But that track is credited to Queen + David Bowie… further, he sings lead in part of the song.
Sandie Shaw recorded a version of The Smiths’ “Jeanne,” with S.P. Morrissey contributing backing vocals.
Oleta Adams (“Get Here”) was a backup singer for Tears for Fears.
Lenny Kravitz sang backup for Sheryl Crow on the track “You’re An Original.”
Don Henley sang backup for Ms. Crow on “It’s So Easy.”
The lead singer of The Shins sings backup on at least two tracks on the most recent Modest Mouse album: “We’ve Got Everything” and “Florida.”
James Taylor sings harmony with Carole King on “Will you still love me tomorrow?”
I’m shaking my head. Once again, a record I thought I had the only extant copy of, and was the only fan of, shows up in another Doper’s collection/awareness.
As an aside, the lovely Linda Ronstadt shows up on a lot of records–er, recordings. She sings on Paul Simon’s “Under African Skies” for one. Which is probably mentioned up-thread, but I haven’t read it all yet.
Are you sure about this? While there are a number of web references that do cite this, I found a few more that say this is a factoid and Steven Tyler never did, in fact, sing backup on this song.
Adding credence to that is this list of Steven Tyler’s recording appearances outside Aerosmith.
The only Left Banke songs I see him credited for are:
“Nice to See You”
“My Friend Today”
“Dark is the Bark”
And Trisha Yearwood was a backup singer for Garth Brooks.
Michael McDonald sang backup on several of Steely Dan’s songs, notably “Peg.”
Nitpick: Aimee Mann is from Virginia, not Canada.
Peter Cetera of Chicago sang backup on Paul Anka’s “Hold Me TIll the Morning Comes.”
A couple more:
Dave Matthews Band’s “Spoon” - Alanis Morissette.
Tori Amos’ “Past the Mission” - Trent Reznor.
Lenny Kravitz did the “ooh ooohs” in Madonna’s “Justify My Love”.
I think that may be a case of somebody half-remembering the facts. Philip Toubus, who played Peter in the movie, never again had a major role in a mainstream film. He began doing x-rated movies under the name Paul Thomas.
There’s been a long standing rumor that Ron Jeremy (aka Ron Hyatt) was also in the movie. Unlike Toubas he was not one of the featured cast; he supposedly was an uncredited extra in the crowd scenes. It’s almost certainly not true; Jesus Christ Superstar was filmed in 1973, when Jeremy was still in college getting his teaching degree.
Trisha Yearwood sang harmony on Emmylou Harris’ “Thanks to You.”
Miss Yearwood seems to enjoys singing backup.
The Eagles seem to be all over the place. Three of them, I think Henley, Frey, and Schmit, are on Bob Seger’s “Fire Lake.”
I have the original 45 somewhere. Love this song. Hoyt Axton may have been a better songwriter than a singer, but he suits this song well. Linda’s backing vocals just make it really special.
On David Bowie’s “Sound and Vision”: Mary Hopkin (credited under her married name of Visconti)
On Elvis Costello’s “The Only Flame in Town”: Daryl Hall
Betty Davis, on much of her self titled album had the Pointer Sisters on backing vocals. (among a ton of incredible talent on that album)
heres a myspace link, note it has sort of embeded sound, it will play one of her tracks when you click but you can listen to up to 4 songs of hers from the link
I was going to mention Michael McDonald and Steely Dan but someone beat me to it 
Dusty Springfield sang backing vocals on Elton John’s “The Bitch is Back”
The great John Prine sings back-up on Nanci Griffiths’s version of his song “The Speed of The Sound of Lonliness”
Bruce Springsteen sings back up (and plays some demented guitar) on Warren Zevon’s “Disorder in the House”
So does Ms. Emmylou.
She produced the outstanding Gram Parsons cover/tribute album Return of the Grievous Angel, and sang backup on at least three tracks.
She also sang backup that just takes Ryan Adams’ “Oh My Sweet Carolina” from a great song to heartbreaking sweetness.