Matthias Wandel has the coolest project ever, a binary marble computer -

It had occurred to me that perhaps with an insane amount of perseverance, it might be possible to build a whole computer that runs on marbles. My second marble machine was however much less based on logic – more on just making lots of noise.

But a few months ago, I had an idea as to how the divide by two mechanisms from my first marble machine could be cascaded together to actually function as a sort of adder or counter. Once I had that idea, I knew I had to try it at some point, and recently, I finally got around to building my marble binary adding machine.

The core of the invention is a modification of the divide by two flipflop to retain the marble that falls off the right side, and retain it until the flipflop is flipped to the left by the next marble. See small diagram above right. The retention of this extra marble allows the state of the marble accumulator to be dumped. The adder would just as well add without it, but the number would have to be read off by the angle of the rockers, rather than have the device dump the count out. Really, if such an adder were integrated into a hypothetical marble computer, reading out the result as a series of marbles would be an essential element.

Binary marble adding machine – Link.