Okay although this is not exactly legal in the U.S. to use. I thought I would highlight it since I know people out there would love to mess around with it, especially at work. Since I work wth two guys who are constantly broadcasting crap about the Red Sox, I plan on building this little beauty myself, and putting it to the test. I will let you know how I make out.
This is the simplest jammer possible:
All you need is one 100Mhz crystal oscillator with an antenna stuck on it. You basically desolder the chip from the PCB and attach the antenna to the contact in the diagram. The antenna should be made from a thin piece of wire and about 80cm long.
This crystal hooked up to a piece of wire will silence a radio for a surprisingly good distance, and through thick walls.
Enjoy, and let me know if anyone has fun with this project.
Update: Here is the original article on the Hack: http://hackaday.com/?s=simple+radio+jammer