Named Phantom Keystroker, it uses one USB port to connect to the target's PC. When it's connected, it will be recognized by PC as a USB keyboard and mouse, and will stay hidden until time comes. Then it will anger the user by typing the odd and random text, moving the mouse cursor randomly and toggling caps lock - all of these combined would ensure user that his PC is infected by virus, or it was hacked. But no antivirus software will locate the source of this mess, because Phantom Keystroker operates on a hardware side, not software. With a set of special buttons on its side it is possible to adjust its behavior and set a time delay, which will be useful to provide an alibi or to prolong the joke. Imagine the fury of the user, when after hours of intensive system clean-up he will discover a strange USB thumb drive, connected to his PC!
While it is meant to be a harmless joke, it is not recommended to use this at work. Of course, it is very tempting to joke like that on your colleagues, but it is wise to keep in mind that such things may not go unpunished. Victim also can become very angry after all these actions, so choose wisely place and time to joke. This device can be purchased for 15 dollars.