Seen recently:
What Would Bubba Do? (On a pickup truck. I do not really understand the significance of this slogan, but for some reason it cracked me up)
And there was one with the universal No sign over a cellphone, and it said:
Are You Driving Or Talking?
No Blood For Oil (on a humungous SUV)
I have seen a picture somewhere on the net of a beautiful Lamborghini, with a vanity plate reading:
PKT CHGE
I wish I could find it again.
“Flight Instructor Carries No Cash”
I’m a fan of “Honk if you passed P-Chem.” Probably not funny to most, but I’d put it on my car.
In Southern California, in my experience, slower traffic really does seem to keep right. Driving through California to Washington, and in Oregon (especially) and in Washington, people seem loth to move over. What’s worse is that I’ll pass someone and they’ll speed up and pass me. Or I’ll move over to pass someone, and they’ll speed up and pace me. No doubt they’re thinking something like ‘You passed me, and now you’re in my way!’
I’m tempted to make a bumper sticker that says:
I USE CRUISE CONTROL
SO IT MUST BE YOU
Seen in Santa Cruz, CA on an old VW bus -
“Jerry’s dead, Phish sucks, Get a job.”
and my favorite (actually on the same vehicle) -
“I beat up three hippies and all I got was this crummy bus.”
Not exactly a bumper sticker, but I had a friend that had a license plate cover that said:
“Women Drivers
No Survivors”
Yes, she was female. And yes, she was a terrible driver. Also, you should have seen the confused looks of the people behind us as they realized we were chicks. 
I don’t know how long they’ve been around, but I saw a yellow ribbon-type “Question War” sticker on the back of a mini-van last week.
I saw a yellow ribbon on a car a couple of weeks ago. Written in fancy cursive script was “Support the Magnetic Ribbon Industry.”
I don’t have kids, but I’ve thought it would be neat to print up a sticker that said something like, “I gave my kid an Uzi”, in response to “my kid beats up your honor student”. It would express my opinion of people who laugh about their kids’ bullying, but probably get my car keyed a lot.
Witch Parking Only
Violators Will Be Toad
My license plate bracket says “TOO CLOSE FOR MISSILES - SWITCHING TO GUNS”.
Does that count?
I’ve seen several “Support America” and “America First” magnetic ribbons on Japanese, Korean, and German cars in the last two weeks.
Not so great now the ribbon thing isn’t as widespread, but I support whatever’s trendy.
I saw
Meddle not in the affairs of dragons, for you are crunchy and taste good with ketchup
I’ve always taken a liking to My governor can beat up your governor.
We’d like to find a bumper sticker for our Prius, that says “I brake for energy.” (Not this one , though… it just doesn’t have the same punch.)
LilShieste
“If You Supported Bush, A Yellow Ribbon Won’t Make Up For It”
My favorite:
EARTH FIRST!
we’ll strip-mine the other planets later.
Saw one yesterday: We Need A Better President
so true
“Cover me, I’m changing lanes!”