Kathy Lee, one of the doctoral students I work with at Stanford, uncovered a product sold by Glakware called Urban Asshole Notification Cards.  As the picture shows, these colorful cards say "Congrats! You're an Asshole" and each comes with a long checklist of different urban
offenses, ranging from double-parking, to cutting in line, to leaving
finished laundry in the washing machine or drier at the laundromat, to
not shoveling your sidewalk. The idea is that hand the card to
offenders, slide it under their doors, or leave on their windshields.
They cost $7.50 for a 10-pack. I am going to order mine right now,
although  I am going to be careful who I give them to, as getting an
asshole mad can be a very dangerous thing.