I now have the most ludacris visualizer setup.
I like iTunes for my music, but the visualizers suck, my computer can't handle them, and iTunes sucks at fullscreen/multi-monitor visualizer support.
So now, I'm playing my music in iTunes, and running Milkdrop2 on my external monitor via Winamp, and using the linein plugin to get a live stereo feed from iTunes into winamp.
EDIT: yeahhhh, this is making my laptop too slow. I guess I'll only use this when people are chilling in my room. Now if only I could figure out how to get the visualizer on both screens. Or better yet, it'd be awesome if someone could point me in the right direction for fixing the iTunes visualizer. Wikipedia says you can get Milkdrop as a plugin for iTunes, but I can't find it anywhere. And no, I'm not gonna switch from iTunes - mainly because I have an iPhone and an iPod, but also because I've been using it for years, and have all my music exactly where I want it, and arranged exactly how I want it.