From the website:
You are not using your windows ? FenÃƒÂªtres Volantes make them fly ! 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.
This screen saver shows off the true power of core video. Sorry kids, Mac only (10.4 systems to be specific).