To discourage the raccoons/possums/stray cats from hanging out under our house (and then crawling inside), TJ created an ingenious little device:
He drilled holes in pvc pipes and stuck them in intervals around the base of the house, then threaded wire through it and ran it with electricity.
Then he tested it on himself (boys!).
We were sitting in the living room later that evening after turning it on and heard a nice 'thump' around the side of the house. I think that means it works. We haven't seen a cat since!