BusKill is a magsafe-style USB dongle that turns off the laptop whenever it's disconnected—like a Kensington lock but much more versatile. It can even, as the name promises, kill the laptop entirely if so configured.
BusKill is the world's first laptop kill cord, a hardware "dead man's switch" that executes a user-configurable trigger when your machine is physically separated from you. BusKill can protect the data stored on your (encrypted) device and any accounts that you're currently logged into from the worst case scenario of a snatch-and-run thief. It is designed to be easy-to-use on Linux, Windows 10, and MacOS. The GUI app currently supports the ability to trigger a lockscreen on all three platforms. Even better, we'll soon be releasing built-in triggers to shutdown the computer when the cable disconnects.
Via Hacker News, where the commenters are all explaining how they would have done it better.