The elementary cellular automaton (a.k.a Rule 110) is a 1 dimensional cellular automaton (this program uses the second dimension to represent time).
To use it download the .zip file unzip it, and inside it, you'll find Rules.html. Open it, and a window will popup asking Rule?, the answer is any number between 0 and 255 (110 and 73 are my favorite), then another popup will appear asking for the world size, 40 is usually good for any computer. Now sit back and watch!