Fiddling with a remote while driving isn't a bright idea, but when stuck in the city traffic everyone's got plenty of time... enough to operate "The Hand" R/C Window Sign, at least. The gadget uses electroluminescent technology combined with animation, controlled by a visor-clipped remote, and can display 7 different gestures.
Although most of them have positive messages ("I love you", "Hang Loose", "Peace"), who wants to bet the others will get much more usage? "Loser" or the all time-favorite middle finger are much more appropriate when dealing with those drivers who think rules are something that only happens to to other people.