Here is a fun tip that I found over at macresearch.org.

• Make sure you have a cool screen saver set (such as 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, bring the terminal window forward and press Control-C (or Command-.)

[Update: If you fear Terminal and the command line, check out bgscreensaver.]