an good one: "marines dont judge terrorists, god does, we just arrange the meeting" my favorite, which i have, which is not on my car due to the hubby being so "aware of appearances"... "how about a nice warm cup of shut the fuck up"
On my bumper "You can't scare me. I have children." And one of my favorites -- on a (former Marine) colleague's laptop "When in doubt, empty the magazine."
Rod's ole dually had a crushed back fender from a gal running into in a parking lot. I put a bumper sticker on it that read, "Driver just does not give a shit anymore!" While we were down in Florida in heavy traffic cars would pull up on the back end fairly close and then back off. Next stop light up they would stay back.
One of my favorite bumper stickers has always been the one I saw quite often at Cherry Point MCAS. It will probably only be familiar to military folks, specifically Marine Corp and Navy servicemen. It said, "The Marines don't need to look for a few good men. They already have them. NAVY CORPSMEN."