The Billiard Ball computer is my favourite.
Basic computing circuits like half-adders can also be made. But making something as powerful as a microprocessor this way would require acres of space - unless your balls or dominoes are very small. Researchers at IBM have experimented with logic circuits that use cascades of atoms bouncing off each other like billiard balls to pass information along their length. Such gates can only be used once, but could be significantly smaller than even the tiniest existing transistors.
