What, no mention of the punchline?
“Nice People Swallow”
Stop Honking I’m on the phone
Caution I drive like you do
Jesus is Coming (look busy)
“Ambivalent People Spit”
I may have to get that, and my license plate is 6022E23.
Really.
And I finally got a real thumping reaction to it from a woman chem teacher just two weekends ago. It ruled.
Hang up and drive.
I are a [name of despised college] graduate!
U.S. out of North America!
First Bank of Dad: Sorry, we’re closed.
Friend of mine in Denton makes these decals. If you go to the various fandom conventions around Dallas (A-Kon, ConDFW, etc.) you will usually run into her selling her wares. She also has a bumper sticker which says that zombies like a girl with braaaaaaaains. 
“Caution, I speed up to run over small animals”
“Entropy isn’t what it used to be.”
I was a soldier for many years. My family lived near a Marine/Navy base where they sold bumper stickers which said: " Navy wives are special".
I thought about buying a couple of hundred of them for soldiers on Fort Bragg.
But my wife would have killed me.
Sighted on the 91 freeway on the bumper of a beat-down 20 year old work truck: “Live Better, Work Union”
A friend of mine is a police officer. One day while driving his patrol car, he couldn’t figure out why so many people were honking, smiling and waving at him. Coming out of a restaurant at lunch, he finally saw the “Honk For Gay Pride” sticker another cop put on his squad car’s rear bumper earlier that morning.
Some favorites:
“Ribbon Awareness”
“Nuke The Gay Whale For Christ”
Batshit/Dingbat 2012! (all the smart people will know who you’re talking about)