Best uncredited features?

I recently found out that Mick Jagger sings backing vocals on "You're So Vain" by Carly Simon (and now I can't un-hear his voice when listening to that song, it complements hers so well).

Obviously EVH's guitar solo on "Beat It" probably takes the crown for the best and most famous example, but what are some other instances of major artists doing an uncredited feature on another song/album?