[Hungry_Myst] has put together this fantastic device to annoy your friends. It randomly emits high pitched sounds, then stops for a while to make it very hard to locate. He has added an extra level of annoyance by putting the noise in vicinity of 17KHz thus making it almost undetectable by people over the age of 30. The fact that not everyone in the room can hear it makes people go even crazier trying to find it.
The parts list is fairly short, and the directions concise. One thing that is fantastic about this article is that he encourages people to improve it. That alone is not a huge deal, but he mentions in several areas specific additions that would make it more user friendly: on/off, pitch control, and delay control.
Related: [jay]‘s Picaxe based Brain Assailant