If up to now, you've been sticking your head out the window to hurl a curse at that other driver, you might want to consider getting the Driving LED Emoticon.
You can stick it on your rear window via a suction cup and let it do its thing. Keep the remote with you and change the display when you want to. Aside from the frowning and smiling smilies, it can also display the words "Back Off", "Thanks" and my personal favourite, "Idiot". I'd sure love to get one and when I do, you know what I'd be flashing.