The Swingle Singers: I remember listening to my mom's vinyl albums (!) of this group from the 1960s, and I was so pleased to see they have continued and are better than ever! Catch them when they're in the States.
Straight No Chaser: I am an unabashed Chaser. You may have seen their special on PBS, "Live from New York." They have 2 Christmas albums, an EP, and a pop Album "With a Twist." Seeing them live is a MUST.