Ian Smith created this supremely cool clock from an old hard drive, some tri-color LEDs and a PIC microcontroller. It works using a simple persistence of vision technique. The disk platter has a narrow slit cut in it and the LEDs are all positioned behind the spinning platter, causing light to shine out through the slit. By adjusting the color of the LEDs as the disk rotates, you can conceivably create any sort of pie-chart style graphics you like, from Pac Man to the hands of a clock.

LED Hard Drive Clock