A Wooden Adding Machine
When I want to add two numbers, I use a calculator. Made of metal. But if you're the awesome woodworker Matthias Wandel, you build a binary adding machine out of wood, then drop marbles into it in order to get your result. BEHOLD, THE WOODEN ADDING MACHINE:
My favorite part is when he gets to the bit representing the number 64 -- the machine's registers literally overflow, resetting the machine's state. You guys, all those years of computer science classes suddenly make sense now.
Wandel has an extensive writeup of the machine's creation, with photos. His main website is a woodworking paradise, including how-to articles on everything from making coat hooks to making a CD changer...out of wood.
(Via The Kid Should See This.)