Lauren McCarthy created the Happiness Hat - a gadget that detects whether or not you're smiling. If you're not, it drives a small metal spike into the back of your head to encourage to you resolve that problem quickly:
An enclosed bend sensor attaches to the cheek and measures smile size, a servo motor moves a metal spike into the head inversely proportional to the degree of smile. Through repeated use of this conditioning device you can train your brain to smile all the time. The device runs on Arduino.
Link via Geekologie
UPDATE 10/29: The YouTube video's status was switched to private, so I swapped it out for a Vimeo version.