Close Harmony Groups:
The Manhattan Transfer
Rare Silk (now defunct, I believe)
Toby Twining - Although, the type of music they perform is on the avant-garde classical side.
Rockapella
The Real Group - From Sweden. My friend, Chris, turned me onto them. Great stuff! They have a song called "Chili Con Carne" which is an instant toe-tapper and smile-inducer.
The Bobs
The Nylons
The Persuasions
I also like a lot of the college groups - Yale Whiffenpoofs, Princeton Tigertones, etc. And I think it's an encouraging sign that the whole male a cappella group circuit is still thriving - and competitive!
-Oh, and when I did The Music Man in high school - I was in the Quartet - I and my fellow School Board Members were made honorary members of S.P.E.B.S.Q.A.! (The Society for the Preservation and Enjoyment of Barber Shop Quartet Singing in America.) -or something close to that.