K'nex computation - using K'nex, a construction toy system, to build logic circuits. For a computer architecture project, these guys built logic gates with K'nex pieces operated by balls, culminating in a 4-bit adder with decimal decoder. It looks like they had more ambitious plans but their project was becoming complex to implement and hard to demonstrate. They have a video of the calculator in operation, and it's also worth going through the archives to read about how bits are represented (by ball position), how gates are implemented (with nice animations), and also their half- and full-adders (also with videos).

Source New Scientist Tech blog.