DOOM Captcha - Captchas don't have to be boring
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.
Squat Captcha - The worst captcha ever! Forces you to do 10 squats
scr.im - Don't share your email on public sites.
Spamty - Spamty is a web service to encrypt an email address and share it on the internet.