G-Force is a highly configurable music visualiser which works with iTunes, Windows Media Player, Google Desktop, Winamp, RealPlayer, MediaMonkey, JetAudio, J. River Media Center and musicmatch.
The program supports multiple visual themes - FractalFlow, In the Club, Infernal, Kaleidoscope, Meditation, To the Stars, Voltage - and they all generate their own gorgeous abstract effects. But you can go much further, tweaking or combining effects like ColorMaps, WaveShapes, FlowFields, Particles, Sprites and more to produce something of your own.
Complicated? Not really, but if you can't be bothered to explore, just tapping the dice icon randomises the parameters and gives you a new look immediately. Keep clicking until you find something interesting, save it as a preset and it's ready for recall later.
If you don't see G-Force immediately, keep in mind that you'll need to be using a supported media player, and be playing music using the visualiser mode, and have G-Force selected as your visualiser.
In the current Windows Media Player, for instance, you would start playing something, then click "Now playing" (the icon bottom-right). Right-click the screen, select Visualisations > G-Force and you should see results immediately. Left-click the screen to see G-Force's options and controls.
This trial download won't expire, but it does display a lot of ads. Upgrading to a commercial edition (from $20) removes these and gives you many more features: more visual effects, extra controls and customisation options, album cover art and title support, the ability to watch G-Force while working in other apps, to use it as a screensaver or use it stand-alone to visualise any audio source.
One of the best and most configurable audio visualisers around.