https://en.wikipedia.org/wiki/Sound_Dues

I think this is a fun historical example. Ships passing through Denmark needed to pay a tax of 1-2% of the value of their cargo. They self-assessed that value.

The twist that makes it interesting was that the King could choose to purchase any cargo immediately at the reported value. If a ship underreported, they might save on tax, but they risked taking a hefty loss.

I have no idea how effective this was, but it's compelling. I wonder whether great self-regulation might need clever design like that example.

The best I've seen is ESRB ratings on video games.

Very few. But it's the core of our current economic system... which explains a lot of the problems.