Famous One Off Collaborations
Examples: Clapton playing the solo on My Guitar Gently Weeps, Duane Allman teaming up with Clapton for Derek and the Dominos, and Stevie Ray Vaughan playing on "Bowie's Let's Dance." There have also been more obscure pairings, such as Brian May teaming up with Eddie Van Halen on an EP called Starfleet, and Sammy Hagar and Neal Schon launching a short-lived band in the early 80s called HSAS. I think of Duane Allman's studio work on Wilson Pickett's version of "Hey Jude, which got Eric Clapton's attention.
The best one, in my mind, is EVH playing on Michael Jackson's "Beat It."
Some more? Leon Russell playing on Sinatra's "Stanger's in the Night?" I think Glen Campbell also played on that too. Campbell with the Beach Boys?