Recently, Google released a new innovative version of reCAPTCHA where it doesn't even request user interaction at all unless Google's algorithm determines that it might be a robot. This is an implementation of this reCAPTCHA as a Kentico Form control, available with full source code. It is based on the New reCaptcha by Štěpán Kozák

reCAPTCHA is a free service that protects your site from spam and abuse. It uses advanced risk analysis engine to tell humans and bots apart. With the new API, a significant number of your valid human users will pass the reCAPTCHA challenge without having to even SEE a CAPTCHA.