I like this hand-crafted wooden adding machine modeled on the digital logic found in microchips. It’s a very simple wooden computer. Still, as simple as it is, it really is showing you how to add any two positive integers that sum to less than 64. Any further addition is just more of the same.
You look at it and you think, whatever those computer circuits are doing on silicon, it just can’t be this simple. But of course it really is that simple. You just need a heaping great crapload of adders to do anything interesting. Which really isn’t a problem when you’ve got a few billion transistors floating around. “Crapload” factors a billion a good many times.
Rambles Blog at Star Chamber 
That is really cool. I must make one.
I’ve seen some prior art:
http://mcsp.wartburg.edu/zelle/cs220/handouts/marbleclock.html
You’re right. That could seriously damage the first guy’s claim on a patent for a ‘binary adding device’. Almost as good as the patent for a ‘stick‘.
Have a look at the Digicomps:
http://www.engadget.com/2006/02/21/3-bit-computer-for-2-bit-nerds/
I still remember “programming” the Digicomp I with plastic straws, clocking it by hand, and watching count in binary. Who knew?