Fenetres Volantes is a nice screensaver that will make your windows fly around the desktop.
After some idle time that can be set in the System Preferences, the windows take off the screen and flutter in the void. On a mouse move, the windows immediately come back and stuck to their original position.
How does it work ? It works as any MacOS X screensaver. Basically, you have nothing to do.
How to install ? You just have to open the .zip file if it has not been done automatically. Then you just double-click on the file, and MacOS X will guide you for the rest of the standard installation.