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.
MTcaptcha - Enterprise Captcha solutions
AuthKong - A reliable captcha service, offers advanced security to protect websites from spam and abuse while ensuring a user-friendly experience for legitimate visitors.
CyberSiara - Leverage the complexity of the human eye to beat bots in a way no other CAPTCHA can. Keep your privacy. Secure your Site. Enhance your User Experience.
CleanTalk - Spam protection for WordPress, Joomla, Drupal, and any other websites