I just discovered this little golden nugget of visualization goodness recently. From a little outfit known as Barbarian Software comes a very cool add-on for iTunes (download for Windows or Mac) that mesmerizes you as you listen to your favorite songs.
Magnetosphere (download it for Windows or Mac) can simulate gravitylike effects that interact with cool, exploding, brightly colored orbs during one song, then switch to swirling green and red ribbons of color for the next.
I like that the visualizer switches its behavior with each new song, providing for a unique experience for each piece of music. A limited control set proves to be more than enough to make the designs expand to overwhelming proportions--simply hit "A" to add 100 particles to the fray or "S" to subtract 100 particles. The developers warn that you should only hit these keys a few times then wait to see the effects on your show. Apparently, getting aggro on the add key can slow the Magnetosphere to a crawl. The developers suggest you turn off the visualizer and start over if this happens, though I haven't had any problems at all today. However, the Magnetosphere is still version 0.1 beta, so we should probably expect a few rough patches to pop up from time to time.
While this is only the very first release of this product the possibilities for development are endless. I hope Barbarian adds more controls in the future with color controls or the ability to switch designs on the fly. Even at these early stages, it looks like this extreme visualizer has what it takes to make it a favorite for all users of iTunes visualizers.