Some hilarious bumper stickers I've seen recently:
Eat More Possum
I live in the South. That's really all the explanation you need.
Alaskans For Global Warming
LOL!!
And my personal favorite: located on the back of an ancient, pinto-like-mobile
DO NOT BUMP
Can't see that without envisioning the classic Pinto-left-fender-explosion scene from Top Secret!