Langton's Ant, created in 5 minutes.
Logic behind Langton's Ant: Every square is either filled (black) or unfilled (white). The ant (purple square) rotates clockwise if it's under white or counterclockwise if it's under black, then flips the color of the square it's under, moving forward one square after all this. The ant will continue working for 10,000 or so steps, until it creates a highway pattern which makes it hit the edge after a while.
