Matthias Wandel built a rudimentary digital computer out of wood. The ingenious Binary Marble Adding Machine is, basically, a 6-bit adder that runs on gravity and uses mechanical flip-flops for memory storage.
[ schematic courtesy of Matthias Wandel ]
Here’s a demo video:
More info on the lovely Marble Adding Machine:
Other interesting creations of Matthias Wandel:
(hat tip: Rick Regan)