We generally use passwords to lock people out of our computers. But Instructables user Thomas_Kirkup found a way to use an actual, physical key -- like the one on your door -- to lock and unlock your PC.
You'll need a little electronics know-how to see this project through, and you'll need to invest in an Arduino Leonardo Board and a key-operated on/off switch from which you can remove the key in either position. If you follow Thomas's instructions, you'll end up with a logic switch that emulates pressing Win+L to lock your computer in one position and entering your password in the other.
It might not be the most secure locking system in the world -- a password is not prone to lockpicks, and there is potential for tampering with the board itself -- but it ranks high in terms of coolness. Of course, you could also use the switch for other things -- the on position could be used to fire up a web browser, start a game or perform a more complicated task, for example. Hit the link to read more.