Hashwall – protection against spam and denial of service without a CAPTCHA

This is what proof of work should have been used for, invented long before Bitcoin. But it seems to never have caught on.

Purelymail uses the system to protect itself from spam: https://purelymail.com

Wikipedia links:

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

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