Malicious USB devices have been covered a lot at security conferences and in the media over the past few years. The Mr. Self Destruct is also worth a look. It is a self-destructing USB keystroke injector that can be programmed to do things on your computer.
You are going to need a ATtiny85, IRLML2502 MOSFET, resistors, and a few other simple components to pull this off. More info is available here.