My friend Pete shared the animated GIF attatched to this thing and said "this has RepRap written all over it", so I decided I had to model it and print it.
This is a model of a Geneva Drive, which converts continuous rotational motion to intermittent continuous motion.. (think of the seconds hand on a clock which "ticks" rather than moving smoothly).
Print the base and then one of each wheel (either printing each wheel independently or use the bothwheels file). The wheel with the finger-hole (wheel one) goes finger-hole side up, The other wheel (with the spikes) goes upside down.
Turn the small wheel with your finger, and it will turn the large wheel in steps.