Microcontroled knock lock door
hackaday.com
This is a controlled lock door by knocks. It works with Arduino. It´s very creative.
hackaday.com
Here is the direct link to his project.
http://grathio.com/2009/11/secret-knock-detecting-door-lock....
This little project really exploded on him, it's been on a bunch of high profile websites in the past couple of days, knocked his server offline, etc, etc.