Here the various cells will be updated depending on the status of the three cells above them (the one directly above and the two neighbors of the latter). Click on the tetraminoes to toggle the rules: the first row represent every 3 cell combination, the cell below is filled if a cell must be created under this combination. Each combination of rules has a number, alternatively you can enter the numbers directly. Check out the wikipedia page for interesting rules to check out. See also the automaton explorer done by Mike Bostock in protovis.