VoiceLive Touch is a great tool for taking a solo performance with voice & instrument, and turning that into a much bigger song. Tom Lang builds up a background loop including acoustic guitar and harmonies. This allows him to sing over top and solo with his guitar. He uses the undo/redo feature with the optional Switch-3 footswitch to bring the harmony part in and out of the performance.
http://www.tc-helicon.com/voicelive-touch