Here's a quick fun tip that I found over at macresearch.org.

• Make sure you have a cool screen saver set (macresearch suggests the RSS visualizer)
• Open Terminal (found in Applications/Utilities)
• Type the following:
  /System/Library/Frameworks/ScreenSaver.framework/Resources/
ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background
• Look at your Desktop.
• To stop the whole thing and return your desktop to normal press Control-C (or Command-.)

There are a few things to look out for with this: Depending on which screen saver you are running and what mac you are running it on, this make take up a large CPU load. Also, your normal background will be shown when pressing F9 or F10 to activate exposé. Finally, if your screen saver starts (from a hot corner or by inactivity for a certain time) it will cancel the screen saver on your desktop.