A friend of mine has these stickers and used one the other night when we were in a crowded parking garage. There's something so satisfying about leaving this friendly reminder for that guy with a fancy car who takes more than his share of a parking lot.
Check out the site:
http://www.iparklikeanidiot.com/. This is the perfect gift for the person you know who is always complaining about the way people park. They aren't permanent so you can have a free conscience when sticking one on a vehicle.