The universal symbol for "I don't approve of your driving" is pretty hard to misinterpret, but for those who don't feel it effectively gets the message across there's the Plate Flipper. The device sits between your vehicle's license plate and the bumper, and at the push of a button on your dashboard the plate will flip down 90 degrees, revealing a message underneath.
Of course that message can be as benign as "Hello" or as road-rage inducing as "Back Off", it all depends on what risk you're willing to take. The website does provide the following warning; "When purchasing our products you take full responsibility for all liabilities associated with the use or misuse of our products" which makes me think the Plate Flipper has already been responsible for some nasty altercation
It's currently available directly from the Plate Flipper website
in a vehicle version for 99.99 and a motorcycle version for 89.99.