reCAPTCHA - reCAPTCHA is a free security service that protects your websites from spam and abuse.
DOOM Captcha - Captchas don't have to be boring
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.
GeeTest CAPTCHA - GeeTest protects your websites, mobile Apps and APIs from bot threats.
scr.im - Don't share your email on public sites.
Humans Not Invited - CAPTCHA that filters out humans and only lets programs pass