Description:

reCAPTCHA is a hybrid mechanical turk and captcha that allows visitors who
complete the captcha to assist in the digitization of books.

From the http://recaptcha.net/learnmore.html manpage:

reCAPTCHA improves the process of digitizing books by sending words that
cannot be read by computers to the Web in the form of CAPTCHAs for
humans to decipher. More specifically, each word that cannot be read
correctly by OCR is placed on an image and used as a CAPTCHA. This is
possible because most OCR programs alert you when a word cannot be read
correctly.