this one’s pretty commonplace in TX:
“SAVE A HORSE, RIDE A COWBOY!” 
I saw a red one that said, “If this sticker is blue, you’re driving too fast!”
If you don’t get it, google “blue shift”
I saw one the other day that said
“You’re driving a car, not a phone booth. Hang up and drive”
“REAL American’s obey the law. Drive the speed limit”
It’s also a pretty irritating song, if you’re interested.
My alltime favorite;
HUMPTY DUMPTY WAS PUSHED
I’ve seen this one a few times on Jeep Wranglers (stuck on upside-down of course)
If you can read this, please flip me over
Okay, it’s kind of stupid, but it always makes me giggle:
Visualize Whirled Peas
Yes, I’m easily amused.
Some folks like mine:
I’m an asshole.
And I vote!
Frodo has failed, Bush has The Ring!
How Would Jesus Drive?
These are on my car right now. Disclaimer: my car is a beater, and desperately needs decoration.
Without our families
Alcohol wouldn’t be necesary
A dirty mind
is a terrible thing to waste
SMILE…
It’s the second-best thing you can do with your lips
If it’s too loud
You’re too old
And a couple more I can’t remember now.
A friend of mine has
Mean people suck.
Nice people swallow.
on his truck.
I kind of want
I see we’ve been visited by the screw-up fairy again. I think that’s the exact phrase.
If I ever get a fargin’ license and vehicle.
I’ve always wanted to print one up in response:
If it’s too loud
You have too much hearing left
One I saw today, a bit funny in its naked gibbering aggression:
“I’d smack you one, but shit splatters”.
On a beater pickup, surprise surprise.
I like “Reunite Gondwannaland”.
The best I ever saw was in a mall parking lot. Somebody had one of those “Shit Happens” bumper stickers and they put it on upside-down.
I want one that looks like “I <heart> my dog” but with a spade symbol instead. I’ve heard of it but haven’t seen it anywhere.
I do have a “Darwin Loves You”.
I saw a great one today: If you can’t figure out how to operate a turn signal, why should I think you can operate any other part of a car.
I want one that says: I’m driving a stick shift, stay off my ass if you like your car.
Also, one that fit my old car quite well: Sorry my car is a piece of shit. My Parents didn’t buy for me.
Heh. Put me in mind of one I saw that said, “I [club symbol] baby seals.” Heh, heh.
You can get them custom-made, you know!