Due to a combination of having a driver's license and being capable of reading at a 4th grade level (I can prove it), I read a lot of bumper stickers while driving. Some are quite clever (See these, for example). A larger number of bumper stickers are ridiculous, preachy, or just plain dumb. For example:
Don't Steal! The Government Hates Competition. - I hope it makes the people who have this bumper sticker feel good to know that Timothy McVeigh had this bumper sticker on his El Camino. What does this mean? People in Iraq during the Hussein regime had the right to display this sticker. In America, it makes absolutely no sense.
If I'd Have Known It Was Coming to This, I Would Have Picked the Cotton Myself. Some bumpers stickers imply that the owner of the vehicle is a racist moron. This one leaves no doubt.
Frustrate a Liberal. Buy a Gun. As a liberal, there's obviously nothing more frustrating than gun ownership. Except shitheads who think that they're the next incarnation of John Wayne, when really they're just too stupid to have a hobby that requires reading.
Misplaced Texan Nothing says "I've had sexual intercourse with a member of my immediate family on multiple occasions." like this one.
Anything from PETA (People for the Ethical Treatment of Animals) I have issues with these folks. There is not another group on the planet that is more hypocritical and morally inconsistent than these assholes. As others have pointed out before and actual research has shown, millions of animals are killed every year in the harvesting of your plants for food, even your $6.99/lb. Organic brussel sprouts. So unless you grow and pick the food, you've got just as much blood on your hands as those of us who use our canines. They also oppose using animals for scientific research, and have even stood behind groups that have made death threats against researchers who use animals in their experiments. Do the whole world a favor and stop eating anything.
Pray is a four letter word you can say anywhere (except in Public Schools). This one represents one of the cardinal fallacies of the religious right. Act persecuted while attempting to persecute others. People pray in schools all the time. What you're not allowed to do under the law is organize a religious service during a school activity or require participation. Christians forget that our forefockers came to this country to escape state-sponsored religion. This one is nicely countered with "Don't pray in my school and I won't think in your church."
You can tell there's a lot of pent up aggression here, so I might start doing this as you send them to me or as I see them. Post them under comments or send them here.