The Tactile Metronome uses multiplexed display circuitry to use three seven-segment digits with only 10 IO pins. It accomplishes this feat by turning the digits on and off rapidly, showing one at a time, but doing it so fast the human eye sees solid digits lit up. To take this picture, I shook the metronome in front of my camera. During the short time the shutter was open, 12 digits were displayed. Pretty cool stuff.
For more details about the Tactile Metronome: wayneandlayne.com/metronome/