Thanks for reviving this thread, Atlanticarc.

The Futureheads' album Rant is all unaccompanied vocals, a lot of them in close harmony. The songs are partly the Futureheads' own, partly traditional and partly 'other' (e.g. Sparks' "Number one in Heaven"). I'm quite fond of their versions of "The Keeper" and Richard Thompson's "Beeswing".

My own 52 Folk Songs project started out mostly unaccompanied & is now mostly not. Harmony singing is something I've only really discovered recently; I've had fun with it, as you can hear on my "Christmas album". in particular.