2 bumber stickers that made me pull over so I wouldn't laugh myself into a traffic accident:

1) In large block letters: I'M THE REAL FATHER (then in small letters underneath) Of Your Honor Roll Student.
Remember My Kid's An Honor Roll student, followed by My Kid Can Beat Up Your Honor Roll Student? I howled!. Not sure why.

2) God Was My Copilot- but then we flew into a mountain and I had to eat him! (with a little smiley face wiping its chin with a napkin no less).

Others that were pretty funny:
WHERE AM I GOING and what am I doing in this handbasket?
CTHULHU FOR PRESIDENT - Why Vote For The Lesser Of Two Evils?
SCULLY/MULDER 2000 - Trust No One Else.