They're (cellular frequency jammers) down to the size of a cigarette pack, and can run off 12V aux power in your car. Range isn't terribly great (~100ft).
Mods: I posted the link (which was the first google result for "cell phone jammer") just to show what the size and cost was. If it needs to be removed, please feel free to remove my comment in its entirety.
A friend has one of these compact models from China, and it works just as you'd expect. It will completely kill any and all cell phone connections (or WiFi, or Bluetooth) within a large room. Apparently it can run for several hours on battery, or the length of a long movie.
Examples: http://www.phonejammer.com/home.php?cat=249
Mods: I posted the link (which was the first google result for "cell phone jammer") just to show what the size and cost was. If it needs to be removed, please feel free to remove my comment in its entirety.