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.
DOOM Captcha - Captchas don't have to be boring
scr.im - Don't share your email on public sites.
Humans Not Invited - CAPTCHA that filters out humans and only lets programs pass
CleanTalk - Spam protection for WordPress, Joomla, Drupal, and any other websites