James Taylor's "That's Why I'm Here": "Playin' 'Fire and Rain' again and again and again..."

Tom Lehrer's "Christmas Carol" sings the first line each of 3 carols (Hark the Herald Angels Sing, God Rest Ye Merry Gentlemen, Angels We Have Heard on High).

And let's not forget all the rap songs that "borrow" the tunes of other artists (the recent one that uses [abuses] "Angel of the Morning" springs to mind).