One I just saw the other day and thought was great:
Honk If You Passed P-Chem
That was the toughest class I ever took, harder than any physics or engineering course. And I was barely entitled to honk.
My friend and i thought of this one
Put on some (insert picture of a t-shirt and pants)
Cover your (insert picture of a dinner roll or two)
It rains, it poors, and then it snows
Fat Hoes In Small Clothes Cover your Rolls
Seen on the driver’s window of a huge rig:
“No Shirt
No Shoes
No Problem!”
God was my co-pilot…but we crashed in the Andes and i had to eat him.
If going to church makes you a Christian, does going to the garage make you a car?
All around me i see nothing but Chaos, Panic and Disorder…At last, my work here is done!
Saw a couple of classics recently. First off this one, which caused me to laugh so hard the woman in the car with the sticker waved to me, smiling:
Support Faith-based Missile Defense Systems
And today, driving around Tucson, I was presented with this thought:
What if the Hokey Pokey is what it’s all about?
Some of my favorites:
“If you’re close enough to read this, I’m close enough to flick boogers on your windshield.”
“WARNING: This car protected by Q Branch!”
“Watch my rear end, not hers.”
“Veni, vidi, VISA – I came, I saw, I shopped!”
and the one I’d love to get. . . .
“BAD COP! NO DONUT!”
My cousin had this one:
“Remember my name, You’ll be screaming it later”
Seen last night on a very well used nissan.
You may soar like an Eagle, but turkeys are rarely sucked into jet engines
Hee ehehe
“I brake for tailgaters”
I like that one because I actually do brake when people are tailgating me.
Whatever doesn’t kill me only points me in the general direction of death. (thought that one up myself, put it on a bumper sticker)
Ok, now I’m scared. Ppl quoting their cousins on bumper stickers like that…
Oh, and I love the “BAD COP, NO DONUT!!”, ewing1.
If it’s got tits or tires, … you’re gonna have problems.
I used to have a fish-n-chips fish on my car, until it broke… I need to get a new one.
Anyway, I too love the “Horn Broken, Watch for Finger.” For some reason that just really, really hits my funnybone.
I like “My Kid Is a Student at University of Michigan. Honk and Throw Money!” My parents used to have one that said “My Daughter And My Money Go To Randolph-Macon Woman’s College.”
And while it’s sort of lame, this one made me snort yesterday, I think because I fell for it (thinking it was serious until I read the second line). It was on the back of some sort of business-style panel van:
Driver Carries No Cash
He’s Married
This is from a Far Side comic (how I loved that strip.)
Two aliens in their little hovering transportation dealy, bumber sticker says:
“Ray guns don’t kill Zorbonians, Zorbonians kill Zorbonians.”
On my car:
GABBA GABBA HEY
“Honk if you hate noise pollution.”
‘Gabba Gabba Hey’? I know that as the cheat code introducer for the classic videogame Descent. Does it have another meaning, or do you just get to be invulnerable with Mega-Wowie-Zowie? 
I never said I liked the guy
Get Drunk and Be Somebody
No more Hiroshimas?
Easy.
No more Pearl Harbors!
And right after the election, in the style of the Gore campaign stickers:
Sore
Loser man
“I might grow older but I’ll never grow up!”
(A Canadian Classic) “In Vancouver you don’t tan, you rust!”
And a classic that a girl from my church has- a Darwin creature being eaten by a fish! I hate those Darwin Critters!