The Digi Comp II is a replica of a binary mechanical computer that can perform basic mathematical functions. As shown by Windell Oskay at Maker Faire Bay Area 2011, it uses billiard balls as its counters and performs calculations in real time as the balls make their transit down a ramp with specially places guides and levers.
http://www.evilmadscientist.com/
http://makerfaire.com/pub/e/5300