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.

Mollom - Mollom is a freemium web service to get rid of spam, unwanted content and fake users on your...

ALTCHA - ALTCHA is a free CAPTCHA alternative that uses a proof-of-work mechanism to protect your website, and online services from spam.

Negative Captcha - A plugin to make the process of creating a negative captcha in Rails much less painful

MTcaptcha - Enterprise Captcha solutions