I thought I'd post a project that I just completed which is perfect for pranking anyone you may know at work or home. This is a circuit board, smaller than most USB plugs, which emulates an USB keyboard. At pseudorandom intervals it sends the keyboard command for hitting the CapsLock key to the computer. So your target assumes that he accidentally hit CapsLock, until it happens 10 or more times. At which point they start trying to diagnose the problem and all the fun starts.
The idea and code came from Garrett over at
MaceTech. His page has links to the source code and schematics. The device is extremely easy to make if you have any soldering knowledge, and cheap to boot. Not counting the circuit board and etching chemicals, the total component cost is under $2 per device. If you don't have any etchant or boards laying around it will run you another $10. The programmer can be made for as little as $5.
A big plus is that since the chip used is a programmable IC, you can change the source code (it's in C) and make it emulate any key on a standard keyboard that you want. This includes Standby, Multimedia controls, and function keys.
Here is a picture of one that I made, plus the board before component placement.
If anybody wants to build one I'd be happy to answer any questions, since the comments section for the original article is just about dead.