"My other car is a Porsche" (which was really funny when I saw this on a Porsche!)

years ago (before any of our tax deductions were born and we had to start driving suburbans) we bought a Porsche C2, and thought we'd be clever by adding the license plate frame that's commonly found on rolling wrecks, which reads "Don't Laugh; It's Paid For"

as for bumper stcikers, around here the fad is to boast a sticker that reads "My Kid is an Honors Student at [insert name here] Elementary School". i almost crashed laughing when i first saw the retort on an old chevy: "My Kid Beat Up Your Honor Student"