hCaptcha - Do you use a captcha to keep out bots? hCaptcha is a drop-in replacement for reCAPTCHA that earns website owners money and helps companies get their data labeled.
ALTCHA - ALTCHA is a free CAPTCHA alternative that uses a proof-of-work mechanism to protect your website, and online services from spam.
GeeTest CAPTCHA - GeeTest protects your websites, mobile Apps and APIs from bot threats.
MTcaptcha - Enterprise Captcha solutions
mosparo - the modern spam protection
AuthKong - A reliable captcha service, offers advanced security to protect websites from spam and abuse while ensuring a user-friendly experience for legitimate visitors.