What? All those posts and nobody mentioned Artisan? (Assuming, of course, that I didn't try to read too fast & missed a crucial reference.) English trio, stunning harmonies. These days they do a small amount of traditional material, the odd English music hall song, hardly any covers (except for an intriguing and innovative version of "The Mary Helen Carter") and a whole whack of very fine original material written by member Brian Bedford. They have several CDs available. Check them out.

Out of the folk realm, Bobby McFerron does amazing jazz-inflected vocal stuff (most people have never realized that "Don't Worry, Be Happy" is an a capella number).

And one last: the Billy Joel thing, "For the Longest Time" is one as well.