I would like to run a screen saver as my desktop background.
How can I do this?
You can look here for instructions.
Essentially, set your preferred screen saver in the Screen Saver preference pane and run this command in the Terminal to see the screen saver on the desktop:
/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background &