reCAPTCHA - reCAPTCHA is a free security service that protects your websites from spam and abuse.
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.
scr.im - Don't share your email on public sites.
AuthKong - A reliable captcha service, offers advanced security to protect websites from spam and abuse while ensuring a user-friendly experience for legitimate visitors.
Humans Not Invited - CAPTCHA that filters out humans and only lets programs pass
MTcaptcha - Enterprise Captcha solutions