Pentatonix is an a capella group with five people. There is Mitch, Scott, Avi, Kevin, and finally Kirstin. This group won the third season of Sing-Off, and has since grown very popular.
The only time they ever perform with an instrument is when Kevin plays his cello. Except for those instances, Pentatonix makes all their sound effects with only their mouths. Here is a link to a playlist of their songs.