I stumbled onto one years ago by accident, maybe an Easter egg or something. I came back to my computer (Mac) after several hours of iTunes playback to see a hitherto unknown visualization running, with fairly primitive-looking graphics by today's standards. It was not any of the visualizations available in iTunes at the time.
I filed a bug on it with Apple and they got back to me asking how the hell I had invoked this, because they'd never seen it before. Never did get to the bottom of it.
