License Plate Flipper
There are few things I hate more than tailgaters... if I had a License Plate Flipper, I could at least send them a personal message. Sure it would be much cooler if the license plate flipped down and machine guns shot up their radiator, a la James Bond, but mere mortals have to compromise. This device also has the very illegal use of evading speed cameras that capture the plate numbers. If you do get caught though, you're pretty screwed.