Behold ThinkGeek's newest product, the Phantom Keystroker. You just plug it into a coworker or friend's computer, tuck it out of sight, and let it go to work. It doesn't need any drivers or anything and will work with any OS. It adds random keystrokes and mouse movements which are sure to make the poor person think their computer is possessed.
It costs $25, which is a bit much, but I really want one. :-)