Wireworld Computer
Wireworld (Brian Silverman, 1987) simulates electron flow with 4 simple rules: Empty→Empty, Electron head→Tail, Tail→Conductor, Conductor→Head if 1 or 2 adjacent heads. These rules support AND gates, OR gates, and universal computation.
Empty
Conductor
Electron head
Electron tail
Click/drag to draw. Select cell type from Draw menu. Load a pattern from the dropdown.