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.
Noisli - Noisli is a fantastic background noise and color generator for working and relaxing. Online soothing ambient sounds like White noise, Rain and Coffee Shop.
MTcaptcha - Enterprise Captcha solutions
Noizio - Feeling that the noise of the big city is too much for you to bear?
GeeTest CAPTCHA - GeeTest protects your websites, mobile Apps and APIs from bot threats.
zenmix.io - Ambient background sound generator