If it were my job to run around in a Barney costume and hug little kids all day, I would want nothing less than an industrial sized air conditioner.
Steve Hoefer at Grathio labs shows off his Arduino based door opener, which responds to knocks on the door bye timing the gaps between the sounds. If the gap matches the programmed knock, it unlocks. Let's never use keys again!
Know a little bit about Arduino and care to make your own? Check out his Instructables post!