Hello! 
I’m the lead developer of mCaptcha, a 100% FOSS CAPTCHA system that uses proof-of-work.
Folks living in oppressive regimes use Tor to access the internet. Using proprietary black box CAPTCHA systems to guard and deny access to such critical resources can be a source of censorship as the black box can impartially deny access. In order to ensure impartial behavior, we should be able to independently reproduce the validation decisions made by guarding mechanisms. mCaptcha is one such protection mechanism, which is transparant and reproducible.
Demo
https://showcase.mcaptcha.org demonstrates a service that relys on mCaptcha for protection. To compute PoW(validation logic), the demo runs JavaScript code. mCaptcha is 100% FOSS, so all source code is freely available at mCaptcha · GitHub.
Requesting feedback
Switching to transparent, libre technologies to protect resources can significantly enhance global access to the internet and privacy in general. So offering impartial, accurate protection for Tor users is is very important to mCaptcha, and I would appreciate any feedback you might have. 
P.S I was going to post on here a little later, but I’m applying for funding with the Open Tech Fund and the reviewer prompted that I make contact. The funding journey is published for transparency purposes, please see here to learn more about it.