ReCAPTCHA
From DocForge
reCAPTCHA is a CAPTCHA system created by Carnegie Mellon University that uses images from books which OCR programs can not read properly. This helps solve book scanning problems while helping prove a user is human.
DocForge, for example, will display a CAPTCHA, provided automatically by the reCAPTCHA system, if a new external link is added by a user who is not logged in.
