There are plenty of times when it would be nice to be able to give other drivers a piece of your mind, or let them know you're having car trouble so they go around instead of riding your bumper.
While there are several gestures that can certainly get the point across, none of them are quite as effective as this rear-window LED marquee.
Robert Dunn of DunnRight Software used an Arduino ADK connected to a trailer light hookup using optocouplers as seen in the circuit diagram below. It receives input from the turn signals and lights and only turns on when the lights are in use because it doesn't have very high visibility during the day.
Robert hand soldered a matrix of 480 LEDs, making the grid sparse so that he could still see through the rear window.
Using the ADK, he can send messages to the cars behind him, and during normal driving, the marquee advertises his personal website's URL.
Check out the videos to see demos of the turn signals and custom messages.