Toxic Audio is a five person (3 men, 2 women) a capella singing group and they are phenomenal. Generally, they do the instrument sounds (principally drums, guitars, bass) as well as the vocals for songs you would know. They also sometimes improvise songs out of audience suggestions, or do rap numbers containing words chosen randomly from books, and a few song parodies. The show is mostly humorous and the live performance eclipses the recordings, which are still awfully good.
You can hear sound clips at:
http://www.toxicaudio.com/
Everything you hear is produced by the human voice.