Media artist Nick Hardeman's audio visualization app Bass Shapes was rejected by the Mac App Store because "it's not useful." So Hardeman released the software as a free OS X download instead. It's a beauty.
The app takes in sound input from your microphone or an external audio source through Soundflower (also free), and the visuals come to life. Watching Bass Shapes, you'd swear that you were seeing a custom, hand-drawn animation that served as some kind of old-school-ish intro to an animated film. But you'd be wrong.
Download Bass Shapes and try it yourself.