Open-source CAPTCHA Alternative
"ALTCHA is a free, open-source CAPTCHA alternative that protects your website from spam and abuse.
ALTCHA uses a proof-of-work mechanism to protect your website, APIs, and online services from spam and unwanted content. Unlike other CAPTCHA solutions, ALTCHA is free, open-source and self-hosted, does not use cookies nor fingerprinting, does not track users, and is fully compliant with GDPR."