XPenguins is a popular novelty application for X11/Xorg (the Linux/UNIX windowing system) which fills the screen with animated penguins. These little fellas parachute down from the sky, walk along the tops of windows, zoom across your toolbar on skateboards etc etc.
By default, JSPenguins only respects the geometry of the browser window. A falling penguin will continue to fall until it hits the bottom of the screen. A penguin moving horizontally will continue until it reaches the side of the screen. This behaviour is easily changed though, and any HTML element can act as an obstacle for the penguins...
The latest version (actually, it's the only version) of JSPenguins is v1.0 and is available here: jspenguins-1.0.tar.gz