There is a simple way having any Mac OS X screensaver replacing the desktop background image.
It is very impressive if you want some animation or nice workflow. That is really cool to make a show of how cool a Mac is.
Just download the small and tiny freeware application Screenie.
It works so simple, that you just click on start and the backround starts to animate. You can stop it anytime the same way.
Here is a short video tutorial I made:
Music: Silent Groove by DJ N-4ceR.