This Pretty cool...

The Driving LED Emoticon is a battery powered, wirelessly controlled message sign that can be attached to the rear window of your car via the included suction cup. A remote control unit is kept in the front of the car (fixed to front window, dash, or laid in a storage compartment) to activate the messages by pressing one of its buttons.

It can display any one of five different messages - smiling face, winking face, "Thanks", "Back Off" and "Sorry". We will, of course, leave it up to you to decide which of these messages suits your particular situation - but we do think it's nice to have a device that allows you to more clearly express yourself to other drivers.

Note: May not be legal in all states. Check your local laws before use.

http://www.thinkgeek.com/gadgets/car/8e9a/