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.
OOPSpam Anti-Spam API - The better, new standard to stop spam content.
AuthKong - A reliable captcha service, offers advanced security to protect websites from spam and abuse while ensuring a user-friendly experience for legitimate visitors.
StopForumSpam - StopForumSpam - a database of known forum and blog spam, its sources and the email addresses reported
GeeTest CAPTCHA - GeeTest protects your websites, mobile Apps and APIs from bot threats.