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.
reCAPTCHA - reCAPTCHA is a free security service that protects your websites from spam and abuse.
AuthKong - A reliable captcha service, offers advanced security to protect websites from spam and abuse while ensuring a user-friendly experience for legitimate visitors.
MTcaptcha - Enterprise Captcha solutions
GeeTest CAPTCHA - GeeTest protects your websites, mobile Apps and APIs from bot threats.
visualCaptcha - The best captcha alternative. Accessible, Mobile-friendly, and Retina-ready! Available for Node.js, PHP, Ruby, Meteor, jQuery, Angular, and vanilla JS.