Do you know what a “CAPTCHA” is? How about a “Completely Automated Public Turing test to tell Computers and Humans Apart.”
Carnegie Mellon University is getting useful work out of CAPTCHA-solving with the Recaptcha project. When you solve the CAPTCHA, you are actually performing OCR on scanned books. The words cannot be accurately recognized by machine-based OCR, but you, as a human OCR system, can do a better job.
Read more at the CMU site or at http://en.wikipedia.org/wiki/ReCAPTCHA