So the built in iTunes visualizer just isn't doing anything for you? Check out Eyephedrine, the visualizer plug-in that makes use of OpenGL 3D. Enjoy the multiple layers blending, realtime cube mapping, reflections, motion blur and light bloom fun across more than 70 visualizations. It also displays album art and track information. Who knew the Crystalline Entity could move like that?
Eyephedrine requires Mac OS 10.3.9, iTunes 4.7, a G4 400MHz or above and a GeForce 2 / Radeon 32DDR, though the recommendations (OS 10.4.3, iTunes 6.x, G5/Intel 1GHz or above, GeForce4 / Radeon 8500 or better) are a bit higher. Eyephedrine is $8US shareware.