It appears that no web page is complete without a Java applet, so here's my obligatory entry in the arena. This program runs the "Life" cellular automaton.

Click within the gameboard to toggle cells for your initial setup, and then hit the "Start/Stop" button. Once you've got your game running, you can adjust cells on the fly, or you can use the button to pause and build more complex structures.

*** You do not appear to have a Java capable browser. Sorry. ***