I learned from Pop Culture Junk Mail of this site aptly entitled Bill Watterson's Private Hell. It's a collection of photographs of stickers on rear-windows of cars that depict Calvin (from the decidedly non-commercial cartoon "Calvin and Hobbes") peeing on various offending brands of (other) cars.
I'm currently living in Windsor, Ontario - the home of the big 3 : Fords, Chrylers, and GM. You see these stickers *everywhere*. I am so glad that this sort of thing has been documented!