It's more or less well known that the stunning visualizer in iTunes 9 is actually a modified version of The Barbarian Group's Magnetosphere. I recently downloaded a copy of the original magnetosphere visualizer, and one thing I enjoyed was the ability to control the amount and intensity of the particles (the glowing, moving dots). The following is displayed in the visualizer's help screen:

+/- Increase or decrease the intensity
A/S Add or Subtract particles (100 at a time)

While these commands are not listed in the help screen of the official iTunes version of the visualizer, to my surprise I found that they still work! They've proved useful for when the iTunes visualizer seems to get stuck for a bit with one of the big black stars blocking the view.