Arduinos occasionally find themselves in projects that are impressive yet impractical. Chris Fenton has admirably demonstrated that with the Fenton Heavy Industries Numbotron. Inspired by electromechanical computing machines of the past, the Numbotron features physically rotating drums for its components. Eight three-digit counters provide 80 bits of memory, an instruction drum offers 560 bits of read-only storage, and the machine can perform four increment/decrement operations per second. The Arduino provides none of its own processing power or memory for this: it simply acts as a control system, replacing unreliable electro-optical state machines to read user input. A video of the machine running a real-world prime-finding program shows just how hypnotic electromechanical devices can be, while the laser-cut chassis offers an attractive housing. For hackers, a simulator offers a chance to try the technology out without the build process.
I just grabbed the Arduino I used in the (now-dismembered) FIBIAC, wired up a quick shield for it, and had it running in a day or two. The nice thing about using a microcontroller is that fancy controls are now pretty trivial, so I was able to speed it up by a factor of 4 or so by carefully controlling the pulse sequencing to accelerate and decelerate the motors without losing any steps.