I saw one in the parking lot of the vet`s that said something like "Real doctors treat more than one species." My friend is going into pre-vet studies in college and I contemplated getting it for her! I also saw one once that said "Dog Taxi" and another that said "If I want to hear the pitter patter of little feet, I`ll put shoes on my cats!"

As for non-pet related:
"The closer you get, the slower I`ll drive"
"I had a life, but my job ate it"
"Money talks - mine says 'goodbye'"