I have a few...

Linda Perry on P!nk's "Lonely Girl". Linda cowrote much of the album, but sings in this track.

John Lennon on David Bowie's "Fame".

Justin Hawkins on Steel Panther's "Party All Day", or on Meatloaf's "Next Time You Stab Me in the Back, You Better Do It to My Face", which Justin also cowrote.

James Taylor on Carole King's "Will You Still Love Me Tomorrow".