Note: I was told that one of those plastic motion detectors in animal shapes would frighten my cats off of counters -- especially effective if I could get one in the shape of a frog.

Uh-huh, nope, not with my kids. Shortly after I set it up in the kitchen, I heard this really annoying 'ribit' steadily for about a half an hour. I was ready to scream and gave up and went into the kitchen. Plastic frog: "ribit". Sassafras, sprawled across the burners of the stove, would thwack plastic frog then return to tucked in position. "Ribit". Sassy thought this was the greatest game ever and that I was a stinker to turn froggie off.

It might work for some cats, but not all.
Someone suggested I leave some pans hanging over the edge of the cabinets so that a leaping cat would overbalance and cat and pan would fall to the floor with a clatter. A variation is to have water in the pan. It might work, but I never had the nerve to try this. [It also sounds awfully messy...]

Good luck.