The Billiard Ball Computer (jameslin.name)

I first ran into this in one of AK Dewdney's books (probably "The Tinkertoy Computer") when I was in high school. I actually took some classes with Tommaso Toffoli as an undergrad but I had totally forgotten that he invented this, so it was only years later when I came back across it that I made the connection.

I've always enjoyed mechanical implementations of digital logic.