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
scr.im - Don't share your email on public sites.
UniqPin - UniqPin is a free for non-commercial use service which destiny improve ability of interfaces detect...
Humans Not Invited - CAPTCHA that filters out humans and only lets programs pass