Back-up Singers

Rita Coolidge was a back-up singer for Joe Cocker on one of his early albums.  Of course, he had Steve Winwood on keyboards and Jimmy Page on guitar on that album too.  No wonder it's such great music.  Most of that band went on to fame and fortune. 

Neil Young used to work with Nicollete Larson a lot, too.  Not sure which songs. 

James Taylor backed up James Taylor on D*** That Traffic Jam.  Carly Simon backed him up on Mockingbird.

Tom Petty and much of his band worked with Johnny Cash on his Unchained album, and they even did a Tom Petty cover!

CSNY guys had long standing relationships with Joni Mitchell and Judi Collins and they all helped one another out often.

Booker T Jones of Booker T and the MGs played keyboard on Willie Nelson's Stardust album.  Ends up they had both moved into the same condo in Malibu about the same time.  Booker T saw Willie Nelson jogging on the beach and called down to him.  They figured out their proximity of address and got together for a jam.  They discovered one another's love for the old classics and had one of their jam sessions recorded.  Booker T Jones did the arrangements, Willie Nelson took lead, and Emmylou Harris sang backup.  It is an INCREDIBLE album if you like the old standards.   Everyone stepping outside the genres that made them famous and jamming to songs that they love rather than trying to be commercial. 

- Zurf

The first one that comes to mind is Sting singing back-up for Dire Straights in "Money for Nothing".  Sting is so prominent thats it's practicaly a duet with Mark Knofler.