A few that come to mind are:
“Honk if you’re horny”
“P.E.T.A. - People Eating Tasty Animals”
During the 80’s Wendy’s came out with bumper stickers with
their slogan “Where’s the Beef?” I saw a few that had been altered
by splicing two stickers together so it read “Where’s the beer?”
And recently I saw this:
“Hang up the damn phone and drive!”
“I hate Barbie. That Bitch has everything.”
“Too close for Missiles. Switching to Guns.”
“My kid’s confidence is sufficient to the point that we see no need to advertise his minor scholastic achievements on the back of our car.”
Where are we going and why are we in this handbasket?
Hug your kids at home
Belt them in the car
12,000,000 miles a minute. It’s the law.
Have that one on your car as you ease past the State Trooper on the highway.
I saw one like that the other day – only instead of using different religious symbols every letter was made out of a cross and instead of saying ‘coexist’ it said ‘believe’.
Another Jesus one I saw a while back…
“Jesus is coming. Look busy!”
In the 1968 election (Nixon v. Humphrey), I saw where someone took
two Nixon stickers and cut one in two, to make: Nix on Nixon.
“If God Didn’t Want Us to Eat Animals
Why Did He Make Them Out of Meat?”
My favorite is one I once saw with a picture of some houses underwater and the words “New Orleans: Proud to Swim Home”.
I saw one today - actually it wasn’t a bumper sticker but a sticker covering the entire back window of a Chevy pickup truck. (Which was being driven by a moron, apparently, not using turn signals, dodging in and out of freeway lanes, vacillating from high speed to below-speed-limit on the freeway) and it read:
TOYOTA: Brought to you by the same people who brought you Pearl Harbor.
Ummm, OK. :rolleyes:
Saw “Honk if You’re Amish” on a car leaving the hockey game the other night. 
SORRY I MISSED CHURCH
I’VE BEEN BUSY PRACTICING WITCHCRAFT
AND BECOMING A LESBIAN
GOD IS COMING
AND IS SHE PISSED
JUST WHEN YOU THINK LIFE’S A BITCH
IT HAS PUPPIES
And . . .
Years ago, the Wisconsin department of tourism distributed bumper stickers that said
ESCAPE TO WISCONSIN
They were everywhere. But one time I saw a bumper that had a couple of those stickers cut up and reassembled to say
NO ONE CAN ESCAPE COWS IN WISCONSIN
Shortly after 9/11: “Dear God, save us from the people who believe in you.”
I don’t know if anybody else would like this or not. I used to have vanity plates that read: Chze Joy. Choose Joy.
Seventy eleben years ago when I was first aspiring to be a writer I used to read Leslie Conger’s columns in either The Writer or Writer’s Digest magazine.
One month she talked about something really bad that had happened in her life, and said that she knew she had a choice as to how to react to it. And she chose joy.
That woman taught me a really important life lesson that I wish I’d learned at my mother’s knee.
We always have a choice. Try to choose the one that truly makes you happy.
Best before: see bottom
I saw one in Texas that said: “I don’t care where your kid goes to school or what his grades are.”
When I was in high school, a kinda major road in my exurb area of Northwest Houston was undergoing a big expansion.
It’s name: FM 1960
FM stands for “Farm to Market” which were old roads that are sort of self explanatory. But they become larger roads, and this one had and was expanding. It impacted a lot of businesses that were located along it and a bumper sticker was available –
I (heart) 1960
Of course, many of the young men in the h.s. band with me cut and pasted them to put on various cases that:
I (heart) 69
Yes, it was heeeeeelarious. Yes. They got in trouble.
BP, that reminds me of the reworked In-and-Out Burger bumper stickers: In-and-Out urge.
Also the Escape to Wisconsin stickers were reworked to “Escape Wisconsin” “Escape to sin” and “Escape sin”
I still like the 2008 campaign stickers that said “Cheney/Voldemort”.