reCAPTCHA is awesome and I'm shocked I haven't heard about it before. This is an absolutely brilliant example of distributing a little bit of work to many people in a socially productive way. Whomever is behind this is a genius and deserves a firm handshake.
In a nutshell, reCAPTCHA provides a free (!) captcha service to third party websites where the captcha pictures are scanned text from books. This turns every captcha lookup into a human driven OCR process. Absolutely awesome.