Jagger on Peter Wolf’s Nothing But the Wheel.
I was thinking the same thing … but it’s a little quirky with Steely Dan.
Steely Dan had only two members – Walter Becker and Donald Fagen. Any other musicians/singers you may hear on a Steely Dan recording would be a hired session player.
The Alan Parsons’ Project was similar – a revolving door of talent, with Alan Parsons behind the sound board as the lone constant.
Tommy Smothers plays guitar and sings on John Lennon’s Give Peace a Chance.
On Robbie Robertson’s great first solo record both the Bodeans and U2 back him on several tracks. Bono sings backup on two songs IIRC.
The king of funk George Clinton sings backup (backwards!) in a Frank Zappa song on one of his last albums. The song was called something like Yahonza.
Tricky I know, but I thought they were officially a band up through Pretzel Logic or so, and then became Becker/Fagen plus the hired help from then on.
AH ha. Maybe one of the Hooples sounds close? I thought he chimed in near the end of the song, bits around the chorus. I’ll see if I can find it online and point to a Bowie-like voice.
On another note (heh), Bruce Hornsby guests on an awful lot of tracks – and has guests on many of his.
David Bowie had some help with “Fame”.
John Lennon helped write this song - he came up with the title and also sang the background “Fame” parts in the high voice. They started working on the song when Bowie invited Lennon to the studio, and Lennon played rhythm guitar on a jam session that resulted in this track. Lennon was one of Bowie’s idols, and they became good friends.
(A bit late on this, but as long as the thread has been revived…)
That was Johnny Guitar Watson, not George Clinton.
The Dictators’ “Faster and Louder”, back up vocals by Bruce Springsteen.
Any famous backup singers on “All You Zombies”? 
Huh. I came in to mention this, then noticed that I had already posted this two years ago. But as long as it’s up, I need to correct my earlier post. It wasn’t Brian Wilson who sang backup vocals on “Don’t Let the Sun Go Down on Me,” but rather his brother and fellow Beach Boy, Carl.
Mrs. Cad has it on her iPod
According to one of the session singers, Tennille and Wilson’s sining never made it to the album.
http://www.thewallanalysis.com/main/frequently-asked-questions.html
I don’t think George Clinton and Frank Zappa ever worked together. Maybe you were thinking of George Duke? Duke performed vocals on Ya Hozna.
She’s not HUGELY famous (though she should be, IMHO) but Darlene Love (“Today I Met the Boy I’m Gonna Marry,” “He’s a Rebel,” “Christmas (Baby Please Come Home”) and the Blossoms sang back up on a lot of songs, like “Johnny Angel.” (And I think “Shoop Shoop (In His Kiss)).”
+1…I’ve always loved that song. And I’ve still got the vinyl LP.
I believe George Jones also did backup on “Running Bear”
A bunch of famous folks including Mick Jagger and Keith Moon contributed handclaps, backing vocals, etc., to the Beatles’ “All You Need Is Love”.
Buck Owens duets with Dwight Yoakam on two songs from Yoakam’s album Tomorrow’s Sounds Today. However, duets is an entirely different thing, I guess… especially these days when all kinds of old musical giants are putting out duets albums.
David Crosby and Graham Nash provide background vocals on the song “Starship” from the album Blows Against the Empire by Jefferson Starship (at that time a “Paul Kantner plus friends” one-off).
There was this whole bunch of people who worked on each others’ albums back in the early 70s – Joe Cocker, Leon Russell, Eric Clapton, George Harrison, etc. For example, Cocker and Gram Parsons both do vocals on the album Motel Shot by Delaney & Bonnie and Friends.
Or “Zombie Nation”?
Jack White sang back-up on “Danger! High Voltage” by Electric Six. I think.
You’re right, it’s George Duke on “Ya Hozna” (actually his vocals from “Sofa #2” turned backwards). I was thinking of another track with backwards vocals, “Won Ton On,” when I mentioned Johnny Guitar Watson above.
Adam Duritz from Counting Crows sang back-up on "Sixth Avenue Heartache’ by The Wallflowers.
Axl Rose was singing back-up for Don Henley on “I Will Not Back Down.”
The Beatles/John Lennon’s “Ballad of John and Yoko” had Paul McCartney singing back-up on the second half of the song. It started out as one word per line, and gradually gets more of the lyrics.
Vanessa Carlton’s voice is the one doing the “Mmmbopbop” on some versions of the Counting Crows remake of “Big Yellow Taxi.”
The B52’s Kate Pierson sang on REM’s “Shiny Happy People.”
Iggy Pop provided backing vocals on Bowie’s Low album eg “What in the World”
The song is “I Will Not Go Quietly.”