The visualizer that is built in to the latest versions of iTunes used to be called Magnetosphere before Apple bought it and rebranded it simply "iTunes Visualizer". Strangely, the number of visualizers that iTunes ships with now is very small, probably in accordance with Apple's declaration that their new visualizer is the last visualizer you will ever need. While I like looking at Magnetosphere, for my tastes it isn't responsive enough to the actual music that is playing.

If you're also left a little cold by the iTunes Visualizer, you might want to check out UsingMac's list of "funtastic" iTunes visualizers. In particular take a look at Fountain Music and Cubism as examples of visualizers that really respond to the music that is actually playing.