Soundstream is a Mac OS X screensaver created with Quartz Composer. It consists of a randomly moving particle generator (reminiscent of Flurry) that reacts to the sound level and pitch picked up by your computer's microphone.
Soundstream looks great when playing music in iTunes, pulsing appropriately to the music.
Also works without a microphone or using the Line In version available on the FAQ page.
This review was originally posted on VersionTracker.com. I'm using it not as a screensaver, but as "wallpaper" on my desktop... instructions are on the developer's site. Very nice example of Quartz's abilities.