The only reason I thought of this topic is because I saw one the other day which I found funny:
Hi, I don’t care. Thanks.
Anyone else wanna share? 
Duct tape is like the Force: It has a Dark side and Light side and binds the universe together
also, almost any one that has been created/modified by the car’s owner.
CAUTION: WE DO DUMB SHIT
Nuke the Unborn Gay Whales.
My favourite was “Prevent inbreeding…ban country music” Haha, bite me, Tim McGraw
**Jamie
Who just happens to be listening to country even as she types this…
One of my all time favorites.
DON’T POSTPONE JOY
The first time I saw this one I was giggling for hours.
**IF YOU REALLY LOVE SOMETHING SET IT FREE
IF IT DOESN’T COME BACK, HUNT IT DOWN AND KILL IT**
My current favorite is:
HANG UP AND DRIVE!
I brake for hallucinations.
Follow your bliss
Why be normal?
I love my country, but I fear my government
I’m a Star Wars fan. Honk if you <insert R2D2 picture>.
I’ve wanted to make one similar to this for myself:
Caution: really bad driver
My child beat up your honor roll student.
You get three:
-
Beam me up Scotty. There’s no intelligent life down here.
-
Eschew obfuscation.
-
A rare one but I say it yesterday:
Too many freaks.
Not enough circuses.
On my car:
Born-again Reincarnationist
Seen on another car recently:
Are you making an appointment with St. Peter on your cell phone right now? right below a Hang up and drive! sticker.
I want to die peacefully in my sleep like my grandpa
Not kicking & screaming like the people in his car
Seen on in the parking garage at school the other day:
I throw peanuts at old ladies.
A big part of me hopes that this car belongs to one of our Jesuit faculty members; I think that would make the sticker even funnier. 
Two that always make me giggle when I see them:
Jesus Loves You. Everyone Else Thinks You’re an Ass
and
Jesus Saves! He Shoots! He Scores!!!
“Support the Right to Arm Bears.” I actually think that one was kind of…weird.
"I Brake For Hobbits"
“Doing my part to piss off the religious right.”
A friend of mine in AZ had that on her car, and I have a picture of it in a collage.