There's a really cool trick on the Mac OS X that allows you to set a screen saver as the background image on your desktop. It's not just a static image from the screen saver, the image moves just like a regular screen saver. It's pretty cool. Here's how to do this:

1) Pick a screen saver that you like.

2) Open a terminal window and type the following:

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

At this point, your screen saver will now be the background image, in full motion. The screen saver will remain as your background image for as long as the terminal window is open.