Captcha's are a known tool to block comment-spammers on blogs or to stop bots from filling using web registration forms. About 60 million CAPTCHA's are solved around the world every day.

The Carnegy Mellon University offers a free reCAPTCHA service, that uses the power of the 'Human effort' needed for filling out a CAPTCHA to translate books and book texts. These are books and texts that cannot be translated correctly by OCR.

But if a computer can't read such a CAPTCHA, how does the system know the correct answer to the puzzle? Here's how: Each new word that cannot be read correctly by OCR is given to a user in conjunction with another word for which the answer is already known. The user is then asked to read both words. If they solve the one for which the answer is known, the system assumes their answer is correct for the new one. The system then gives the new image to a number of other people to determine, with higher confidence, whether the original answer was correct.

The reCAPTCHA service can be used for web-forms, Email protection and a wide range of programlanguages and applications.