There must be hilarious undiscovered unknown rube Goldberg machines out there where a human completes a captcha, then the host sells the captcha to the seller who passes it to next user who passes it to the next website who sells it again and so on.

POW captchas aren't actually captchas, it's just hashcash (IE make sure the person reading the content is using as much or more compute as you are serving it so they can't DOS you either on purpose or accident.) We stopped needing it for a while because compute and bandwidth grew really fast while serverside software mostly stayed the same.

Agreed, it indeed is Hashcash. I love it. So simple yet effective.

http://www.hashcash.org

https://en.bitcoin.it/wiki/Hashcash

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

C implementation (feature-rich): https://github.com/hashcash-org/hashcash/tree/master/c

A Factor (Forth-like language) implementation of it: https://github.com/factor/factor/blob/master/extra/hashcash/...