A couple of years ago inventor Steve Hoefer wrote a how-to project for MAKE called the Secret Knock Gumball Machine. You record a special knock pattern by rapping your knuckles against the case. After that, only people who enter the correct knock pattern get a gumball.
Recently Steve made a Secret Knock Drawer Lock and wrote a tutorial for Adafrut's learning site.
Steve remarks, "I've built this project a dozen times and every single time I finish it, it makes me incredibly happy to see it work. Even though I know every detail of it, it still feels a little bit magical." Indeed!
Below, a video of the lock in action.