MIDIJam (aka Animusic Lite)

[The Pezman]

I'm surprised this hasn't come up.

http://www.boardgamegeek.com/thread/118705

So it creates a visualization of the appropriate instruments when it's fed a MIDI file. Again, much like the famed Animusic video but nowhere near as high end. The only problem is that you don't have any control during the song. You sit it through and watch the camera pan from instrument to instrument. But for a free product, I think it's absolutely amazing, and fun to look at.