I have no bumper stickers on my car but my guitar case is festooned and I have some on my bulletin board at work.

Some of my favorites:

Question Authority
Question Assumptions
Aroostook County - The Way Maine Used To Be
I Read Ruth Moore
Born Again Pagan
Save Barney From Newt (This one is dated but will be back in style soon)

"Born Again Pagan" is on my bulletin board and I expected it to raise some eyebrows, especially those of the conspicuous Christians. Unfortunately it has been ignored for years.