Bumper Stickers are great places to find humor in the form of quips and puns.
Let's try to keep this to ones that you've actually seen, not just some collected from a list somewhere.
One of my brothers had one back in the 70s that his print shop teacher at Archbishop Carrol High School had created; it was quickly picked up and copied before he was able to copyright it:
ILLITERATE? Write for free help!
One that I've seen several times over the years that I'll have to print for myself if I can't find it:
Driver carries no cash -- he's married
One that I saw in Washington DC a few weeks ago:
Terrorist Hunting License. License No. 0911. Open Season, No Limit.
And finally, one I saw on the Baltimore Beltway the other day:
Rehab is for Quitters