The lowest known form of humour?
The Monger weighs in briefly on bumper stickers. He cites his all-time favourite as, "Earth First - We'll Log the Other Planets Later". This is indeed a rare beauty (in the Bob-and-Doug sense).
You really have to wonder, though, about people who actually put these things on their bumpers, given the essential permanence of the act. Even funny TV ads get stale after the first 10 viewings; they get rotated out within weeks or months. What kind of person effectively tells the same joke over and over, day after day, even to people who have heard it many times before? I Love Alberta Beef is one thing; it's a cause you support, and you are likely to support it well into the future. Good humour simply does not lend itself to bumper form.
That said, there a still a few that make me chuckle, so long as it's been awhile since the last sighting: the rudely-spliced "Honk If You're Horny For Jesus", the nonsensical "My Karma Ran Over My Dogma", and the defiant "Don't Like My Driving? Call 1-800-EAT-SH*T".
My present favourite however, given my home city and its most famous resident, is a gloriously insensitive boast, distributed by a local radio station last autumn:
"I TOOK DAR TO VEGAS"