A device that waits for someone to knock on the door and then knocks back with the same sequence of knocks.
A digital alarm clock re-purposed to display the time as a binary number. All the original electronics were removed and replaced with custom circuitry including an AVR microcontroller, display driver circuit, power supply, and a two-tone audio amplifier with speaker.
A dwarf-hamster ball using sliding weights, servos, gyros, accelerometers, a compass, and an Arduino microcontroller to roll the ball around. It uses a unique propulsion mechanism such that there is no rotational axis, it can go in any direction at any time.