My favorite is an old one but it goes like this:

It will be a great day when schools get the money they need and the military will have to hold a bake sale.

"If I had known grandkids would be so much fun I would have had them first."

The ultimate anti-social cause bumber sticker:

"Nuke the Gay Unborn Whales."

My Dad had two which always cracked me up:

If you can't run with the big dogs, stay on the porch

If everyone liked the same thing, there would be a line around the world

A couple of old ones:

On the bumper of a Volkswagen: When I grow up, I want to be a semi.

And when deadly rival of the University of Kentucky, the University of Louisville, won the NCAA: First in the nation; second in the state.

Wanted--training wheels for mountain bike

Saw this one yesterday:

"When Clinton lied, nobody died"

Bumper sticker with a picture of George Bush and the statement:
"No Millionaire Left Behind..."

I guess if we're going to get political, I prefer the other Clinton bumper sticker: When Clinton lied, honor died.