There's a utility for Thinkpads running GNU/Linux that lets you execute commands by physically knocking on the machine. This registers as activity on the accelerometer built into the laptop (used to park the hard-drive heads in the even of a fall) and is translated into commands within the OS.
For the first time, you can hit your computer and get a meaningful response! Using Linux and the Hard Drive Active Protection System (HDAPS) kernel drivers, you can access the embedded accelerometers on Lenovo (formerly IBM) ThinkPads, then process the accelerometer data to read specific sequences of "knocking" events — literally rapping on the laptop case with your knuckles — and run commands based on those knocks. Double tap to lock the screen, and knock in your secret code to unlock. Tap the display lid once to move your mp3 player to the next track. The possibilities are endless.
(via Make Blog)
