http://www.audio-surf.com
Takes your songs and converts them into a track based off frequencies, tempos, bass rhythms and other various factors. You have a variety of options for gameplay, but most of them are quite fun.
http://www.youtube.com/watch?v=BUn0uAeNNzY
That's a link to a video of me playing One More Lovely on Mono-Pro. The quality isn't amazing but you get the picture from it.