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.
Captcheck - Open source reCAPTCHA alternative
UniqPin - UniqPin is a free for non-commercial use service which destiny improve ability of interfaces detect...
CyberSiara - Leverage the complexity of the human eye to beat bots in a way no other CAPTCHA can. Keep your privacy. Secure your Site. Enhance your User Experience.
mosparo - the modern spam protection