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!
From submitter azgli: The speaker system at a friend's party was missing a proprietary 13-wire cable between the main unit and the speaker controller. This was my solution to the problem - 13 individual wires insulated with Scotch tape! More Scotch tape kept them from shorting when the sub started rattling things around.