Secret Door Lock Knock Detector.

Gizmodo:
DIYer Steve Hoefer has been hard at work making every 6 year old's dream come true: A device that unlocks your door whenever someone seeking entrance taps out the secret knock.
His device, which fits easily over the inside of your doorknob and lock, counts the time between successive knocks and can be reprogrammed to whatever knock-pattern you see fit. If the wrong series of knocks is tapped out, the device simply ignores it and waits for the appropriate pattern to allow entrance.

Oh man, I really want one of these. I'd like to put it in a cooler form factor. The PVC isn't doing it for me. Check out the original link here.