This post was published more than a few years ago (on 2005-03-07) and may contain inaccurate technical information, outmoded thoughts, or cringe takes. Proceed at your own risk.
Optical Alchemy has a wicked keen iTunes visualizer— Jewelcase, which displays a 3D rendered spinning jewelbox of whatever album your currently playing song is from, complete with tracks listing and cover art. Of course, it uses your cover art in iTunes, so it works best if you’ve loaded in all your cover art (or got it with your AAC files from the iTMS).
Unfortunately, it doesn’t seem to work with the new Intel Macs. I donated the developer five bucks to keep working on it, but I’m sure his time is worth more than that. Ah well, it was fun while it lasted.