There's another one I've been thinking about for years.... now, this one ain't holy, so go ahead and roll your eyes... lol.
I've always wanted a device to sit on top of my car (picture the little "taxi" sign that lights up on top of taxicabs) that could flash different messages to drivers when they do something to tick me off... for example, someone is driving like an idiot, and I pull in front of them, slow down, and flash my "in a hurry?" sign.
Or I daydream at the light, so when it turns green, someone beeps at me. I'd use the, "awwww shuddup sign."
Some of the other ones would say stuff like:
You're just an absolute dummy.
I hope you're on your way to driving school.
Did you get your license from a CrackerJack box???
You drive like an idiot.
So THAT'S how your car got all banged up.
Calm your nerves, I see you.
Beeping doesn't make me go any faster.