About
Visualizers are employed within iTunes.

Apple's own Music Visualizer protocol works by means of placing one's protocol formatted composition within the Composition Repository Library/Compositions, either one's Home folder ~/Library/Compositons or /Library/Compositions.

As stated earlier, one can combine protocols within the same composition, so it can be used as both a ScreenSaver and a Visualizer. So long as the composition is saved both within the Composition Repository and a copy saved in the ScreenSaver folder, then such a composition would be available both within iTunes to visualize audio and as a screensaver too. One cannot combine Image Filter protocol with any other protocol based composition although Visualizer based compositions can combine any other protocol than Image Filter.

There is a third location that acts as part of the Composition repository, which is /System/Library/Compositions - reserved for core system compositions, given that these are exploited by Finder and other default applications, it is advisable to leave these alone.

There is nothing to stop one examining these compositions and exploiting them either for all users , or just for oneself. You just have to remember to save them outside of the System/Library/Compositions folder.

Some of the best places to start looking into Music Visualizers is with Apple's own Quartz Compositions, the Developer's Music Visualizer template and Jelly, one of three visualizers that come pre-installed with iTunes. This short tutorial is an introduction to music visualizers.