If you like to mess with people the aptly named Annoy-a-tron over at ThinkGeek will do the trick. This little device is magnetic and will stick to metal surfaces allowing you to hide it for more sick thrills.

What the device does is beep at irregular intervals driving your co-workers and friends crazy while they try to find what is making the noise. You get three noise options 2 kHz, 12 kHz, and alternating. I’m not sure what a kHz sounds like, but I expect it is annoying.
The beeps sound off at 2 to 8 minute intervals. ThinkGeek says the 2 kHz sound is annoying but the 12 kHz is higher frequency and will really get on someone’s nerves. The device runs on one CR2032 battery that is included and measures in at 2.5” x 1.25” x .3” and runs for 3-4 weeks on one battery. Image via ThinkGeek