These guys have built a basic computer within the Wireworld cellular automata system.
Lemme break it down.
A cellular automata is a simple kind of computer simulation. Imagine a grid in which each cell can be on or off. At each step, every cell decides to be on or off depending on its surroundings. Repeat. Despite the simplicity of the setup, complex behavior often results.
Wireworld is a specific kind of cellular automata, in which there are four kinds of cells and more complicated rules. The system lets you draw wires and electrons, with electrons traveling as time steps pass.
By drawing very specific configurations of wire cells, Moore and Owen were able to create simulations of the basic building blocks of digital electronics: diodes, logic gates, flip-flops, and memory.
Then, they used the components to lay out a computer.
(via Rudy Rucker)