Today, we are officially introducing a proof-of-work (PoW) defense for onion services designed to prioritize verified network traffic as a deterrent against denial of service (DoS) attacks with the release of Tor 0.4.8.
A new DoS protection mechanism for Tor leveraging Proof-of-Work.
This isn't the first proof of work "captcha" system, there's several already out there.
This one, however, is built right into the protocol.
https://friendlycaptcha.com/