Google reCAPTCHA
From GCA ACT
Description
Google reCAPTCHA protects websites from spam and abuse. It uses advanced risk analysis techniques 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 solve a CAPTCHA. reCAPTCHA offers more than just spam protection. By turning on the CAPTCHA, you can help digitize old books and newspapers.