Is it possible to show a screensaver on a secondary monitor while using the primary monitor?
You can set your screensaver to be your desktop background (link to source).

To do so, run this in the Terminal:

/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background


This works on both monitors (tested just now in Lion), so as long as you don't put any windows on the other monitor, it'll just show the screensaver. (Unfortunately, the screensaver will also be behind all of your windows on the screen you're using. I hope you really like your screensaver.)