A red bumper sticker: “If this bumper sticker is blue, you’re going too fast.”
“186,000 miles per second–it’s not just a good idea, it’s the law!”
I saw this one about thirty years ago, and I still remember it (which I can’t say about most other things I saw thirty years ago):
“Happiness is all right in theory, I guess…”
It was hand-written.
I saw a great one today. It had a big arrow pointing to Michael Jackson’s current “face” and read WHAT THE FUCK IS THIS!?!
Get in. Sit down. Shut up. Hold on.
Oh my Jesus, what’s funny is just hours ago I was thinking to myself that there should be a bumper sticker that reads “My other car is a <insert crappy car name here>” bumper sticker. Then I thought about it some more and decided against it, as nobody who buys an expensive car would want to desecrate it with cheesy bumper stickers. Maybe I was wrong.
Aaaaand I used bumper sticker twice in one sentence. Scratch the second one. Seems like I can’t get everything right in one post. I’m a :wally
A friend has this one on her truck:
Of course you can trust the government. Just ask an Indian
I don’t have a license to kill. I have a Learner’s Permit.
On my brother-in-law’s truck…
On an older vehicle he once had, “Nuke Gay Whales for Jesus”. He said it was to piss off everyone worth pissing off.
Some women are born leaders. You’re following one.
I may be slow but I’m in front of you.
Save the planet, recycle an environmentalist.
Spiritual fruits, not religious nuts.
Jesus loves me, but can’t stand you.
God created Adam first, saw the mistake and created Eve.
From the 70s :Jesus Saves-Esposito scores on the rebound!
On my truck I had 2 home made bumper stickers:
I’m an Asshole, And I Vote!
and
Free Tibet
Limit one per customer.
While supplies last
Not valid with any other offer.
Somebody attempted to steal, or they just were offended by, the “vote” sticker a couple weeks ago, So now I have to make a new one.
Hah! Love the Tibet one, Nurse Carmen. 
My only bumpersticker at the moment says ‘geek.’
Being a unix geek, my old favorite is the “got root?” one. Another is “I read your email.”, in answer to the many times I get asked that very question.
MY OTHER CAR IS UP MY NOSE
“I love vegetarians, they’re delicious!”
“Those of you who think you are right are annoying to those of us who are.”
and my all time fav:
“If at first you don’t succeed, ski diving is not for you.”
Another one for the physicists:
“Positrons are Another Matter”
I got that one quoted on Paul Harvey years ago…
I saw this one in Clackamas, OR.
Do not meddle in the affairs of dragons,
For you are crunchy and taste good with ketchup. 
“I brake for animals - I kill them, I eat them, and I wear their skins”
“Jesus is coming back, and will he be pissed when he sees this mess”
I’ve seen a similar one:
“Jesus is coming, look busy”
Gas, grass, or ass… No one rides for free.
Despite really disliking drivers who follow too closely, I love the bumper sticker that says:
I’m not tailgating, I’m drafting.
I always liked the old Barry Goldwater bumpersticker:
AuH2O
Also,
Engage in Random Acts of Senseless Kindness
I saw one on the way home from work and I thought about this thread.
Skydiving. Good to the last drop.