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.
CleanTalk - Spam protection for WordPress, Joomla, Drupal, and any other websites
reCAPTCHA - reCAPTCHA is a free security service that protects your websites from spam and abuse.
Akismet - Akismet is a spam fighting service that protects millions of WordPress sites from comment and contact form spam.
UniqPin - UniqPin is a free for non-commercial use service which destiny improve ability of interfaces detect...
visualCaptcha - The best captcha alternative. Accessible, Mobile-friendly, and Retina-ready! Available for Node.js, PHP, Ruby, Meteor, jQuery, Angular, and vanilla JS.