Nice cfq83! But I was wanting one where you could make the LEDs display text on demand. Like if someone is being a dork behind you, you could switch on the LED screen and type a custom message to them instantly (a phrase more creative than giving the finger).
At the Pep Boys near us is a full size LED scrolling thingy for $40. It is maybe 12 characters wide and very visible. It will store 5 120 character messages. It has an infrared remote control. I don't think it has a keyboard, though, so you would have to be very prescient about what you might want to say. I already have a yellow-cheapo-plastic cutting board in my car with the words "THANK YOU" on it. It's good at disarming people's aggressive driving personalities. I didn't make the insulting kind in order to avoid the temptation of being road-raged.
I would probably have these (mild) messages :
1 - 38 TOTAL MPG, THANKS GASSAVERS.ORG!!!
2 - FOLLOW ME FOR GREAT MPG!!! (to disarm the tailgaters)
3 - THANK YOU! (when someone is nice)
4 - PLEASE? (in conjuntion with a turn signal)
5 - DANGER!!! (when I see crap or accident in the road ahead)