Catwatch Cat Deterrent
I've bought 2 of these so far, and both are working well. I put mine against their favorite "toilet bush" and their obnoxious smell vanished within 3 days. (Before Catwatch, I rarely saw a cat, we could just smell they'd "visited"!)
I sent the second to my friend and she reported watching a cat sit on her fence, debating, before deciding not to come any closer. Job done.
A simple visual check (Does the red light come on when you go near) means it's easy to see when the battery needs replacing.
Only one drawback - squirrels can't hear it. But there's probably a different version for them if you really want to keep them out.