Conway's Game of Life
Conway's Game of Life is a cellular automaton: a grid of cells lives or dies each generation by a few tiny rules, and from that simplicity comes surprisingly lifelike behavior. This is my take on it — living cells glow as neon dots that spring into existence, melt away when they die, and shift through a cool aurora-to-magenta spectrum as they age. Moving structures leave phosphor trails, so you can actually see things travel, and the board starts with a Gosper glider gun endlessly firing gliders across the field.
Click or drag on the grid to paint cells. The rules: a living cell with two or three neighbors survives; a dead cell with exactly three neighbors is born; everything else dies.