As noted in our iTunes 8 first look and review, iTunes 8 features a new default visualizer that many people--myself included--find much more visually interesting than the version in prior releases. As I noted in the First Look, after enabling the visualizer (View -> Show Visualizer or Command-T), you can call up its help screen by pressing the "?" key. On the help screen, you'll see that you can press M to change modes, P to change palettes, N to toggle nebula mode on and off, and much more.
But you won't find the following keys listed in help--these keys are left over from the days of yore, when the now-in-iTunes visualizer was an independent iTunes plug-in known as Mangetosphere. These hidden keys offer even more control over the behavior of the visualizer:
+ and -: Increase or decrease the intensity (brightness) of the particles; multiple presses further increase or decrease the intensity.
A and S: Add or Subtract particles to the visualizer. You can make the visualizer as complex (or sparse) as you wish.
R: Reset the intensity and particle count to their default values.
E: When in nebula mode (press N), this greatly accentuates the nebula clouds, making them very easy to see. (If you've used the M key to change modes, you may find that the nebula clouds aren't visible; it seems they're only used in certain modes.)
There may be other commands that aren't covered in the help screen, but these are the only ones I've run across so far.
This story, "Secret Controls for the New ITunes 8 Visualizer" was originally published by Macworld.