A friend gave me a bumper sticker that reads “Republicans for Voldemort.” I display it proudly in my back window next to the “Hogwarts” one.
Impotance…Natures way of saying…"No hard feelings"
Stupidity should be painfull.
or
Real men drive cars from the 80’s
Of course. 
On the road yesterday there was an old, beat-up Chevy Celebrity with part of the front bumper missing, a number of rust holes, dents, scratches, etc. I’ve seen cars in pictures from Baghdad that were in better condition. The bumper sticker read:
YUP, IT’S GOT A HEMI
Gotta admire the guy’s sense of humor. 
I’ve always liked: “I started with nothing, and I still have most of it left.”
We are Microsoft. Resistance is futile.
The sticker had the MS logo on a borg cube.
OK, this is an old one, but it’s still my all time favorite bumper sticker ever.
In case of Rapture, can I have your car?
I was behind a truck recently when I noticed a small amount of text arranged on the upper-left portion of its tailgate. Upon further inspection, it was a small bumper sticker bearing the following inscription:
up, up, down, down, left, right, left, right, b, a, select, start
Awesome.
In a related vein, I saw a sticker that said “Where do you want to go tomorrow?” with the “microsoft finger” pointing to heaven. It was a promotional bumpersticker for a Billy Graham promotional speaking gig. I’m usually down on “preachy” stuff but I thought that was actually pretty clever.
Not a bumper sticker, per se:
On a beat up little Festiva, sitting outside a lumber yard, the guy drove it was using it obviously as a mobile mini tool shed for construction.
Where the name plate on it should have read: Festiva. Had **Dodge Ram **.
It was very funny to see that.
A pentagram with the text:
“Give me some of that that real old time religion.”
I see one at school all the time that says I bet you’d drive better with that cell phone shoved up your ass.
Hanh? Home video game control panel? National Association of Easily Distracted Square-Dance Callers? Assembly instructions for a child’s bicycle, translated from Japanese to Urdu to English?
Not really a bumper sticker, but there’s a picture floating around online of a Lamborghini with a vanity plate: “PKTCHGE”.
To be more precise, the Konami cheat code (IIRC, it’s not just for Contra).
I saw one about a week ago that said “Jesus loves you, but everyone else thinks you’re an asshole.”
I saw one on a lowrider truck the other day that said PRAISE THE LOWERED!