"Joan of Arc" with Leonard Cohen duetting with Jennifer Warnes on the "Famous Blue Raincoat" album

did backing vocals on Dionne Warwick's version of "Will You Love Me Tomorrow", IIRC.

Their debut single is a mash-up of Under Pressure and Ice Ice Baby featuring the man who originally released the latter, Mr Rob van Winkle, AKA Vanilla Ice.

"You Just Haven't Earned It Yet, Baby" has Johnny Marr playing guitar on it.

Bryan Ferry also got him in when recording "The Right Stuff", which was a "cover" of the Smiths' instrumental original with words and music by uncle Bry.

... had some of the original artists playing on his covers on the American Recordings albums. Tom Petty plays on "I Won't Back Down", for instance.

did a version of the classic Jam b-side Shopping a little while ago with Weller on backing vocals.

...joined Aztec Camera on their cover of (If Paradise Is) Half as Nice.

Singing The Man Who Sold the World.

Whilst reading this very thread, Lily Allen performing 'Straight To Hell' with Mick Jones came up on shuffle...

Gulp.

Ozzy sings on Primus' cover of NIB

Robert Downey Jr (no, really) recorded a cover of Yes' Your Move, with Jon Anderson yelping away on backing vocals

Pete Townshend features on their cover of "Substitute", from the album that one of my friends refers to as "Odor Eaters"

but I think The Beatles appear on virtually every Oasis song.

sang and played guitar on Arcade Fire's Life On Mars.

...Neil Young pops up on Emmylou Harris's haunting version as a backing vocalist.

Laughin' Lou Reed used to pop up on a ginormous video screen during the Zooropa tour to help Bono warble 'Satellite of Love'.

from the Blue Nile sang on Craig Armstrong's version of Let's Go Out Tonight.

Labi Siffre is in the video for "It Must Be Love" if that counts?

Features Down By The Seaside by Tori Amos & Robert Plant

Walk This Way featuring Aerosmith

pops up on Bowie's much-maligned cover of 'Across The Universe',

Joined Arcade Fire for their 'Five Years' run through.

Lou Reed joined in with Heather Small and the gangs massacre of perfect day.

..instances of this on Nanci Griffith's "Other Voices,Other Rooms album.John Prine lends backing vocals to Nanci's version of "Speed of the Sound OF Loneliness",Whilst The Zim wheezes away on his gobiron during "Spanish Boots of Spanish Leather".And there may be others,but I can't remember.

....has a Tom Waits covers album called "Wicked Grin". Waits produced some of it and plays and sings on it too.

Doesn't Waits also sing on Springsteen's cover of "Jersey Girl" on the live album?.......I may be thinking of a bootleg though.

Kirsty MacColl does backing vocals on "They Don't Know About Us" by Tracey Ullman.

Randy Newman plays piano on Harry Nilsson's "Nilsson Does Newman".

Duke Ellington plays pedal steel on Steely Dan's cover of "East St. Louis Toodle-Oo".

Cover of Roadhouse Blues featuring one R. Krieger on axe..

I just checked this c/o Mr Google and.. bugger me, there's a vid on YouTube!

I've not seen this before. It's the ETL version. Cracking.

On the Shrek soundtrack - ostensibly by the implausibly named "Butterfly Boucher"is a duet with Bromley's finest...

has The Coolest Man On The Planet joining her singing.