I have never been one for screen savers, however I've recently found one that I like quite a bit. Using a little thing called ScriptSaver (previously used in this hint), which runs an AppleScript of your choice when your screen saver would activate, I wrote a short AppleScript to start up iTunes visualization if a song is playing. This could be extended to do a plethora of things, such as start a specific playlist if nothing was playing, or play a random song, etc. But as I said, I don't like screen savers much, so I only want it to come on if I'm listening to music but not using my machine. For that, this is perfect.
tell application "iTunes"
if player state is playing then
set full screen to true
set visuals enabled to true
end if
end tell
[robg adds: This worked fine on my machine; if you have more than one display, the visuals will play on the monitor that holds the iTunes window.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040205044431199