I've got several industrial sewing machines mounted on leveling feet to deal with my uneven floors. But I've found the vibrations of the machines are enough to loosen the nylon-insert lock nuts attaching the feet to the table legs. While poking around for a better solution, I came across this Nord-Lock Washer design:

It's a clever exploitation of wedges, and is of course completely reversible. The idea isn't new; the company has been making these for some 30 years.

Nord-Lock's newer X-Series of washers feature serrations on both sides, allowing them to bite into the underside of the bolt head:

While the washers are removable for the sake of performing maintenance on whatever you're bolting together, the end user has to make the call on whether they're reusable or not:

They're also not quite idiot-proof in that one must ensure the washers are laid down in the correct orientation. But overall I think it's a pretty sound design.