Xdesktopwaves is a cellular automata setting the background of your X Window System under water. Windows and mouse are like ships on the sea. Each movement of these ends up in moving water waves. You can even have rain and/or storm stirring up the water.
An interesting little experiment. Compiz does a better job of water simulation for everyday desktop use, but if you like playing around with programs, you might like xdesktopwaves.