This free service from Carnegie Mellon University requires others to answer a two word puzzle before revealing your email address to them. There's also an audio option for visually impaired users.
Other than protecting your email from spam bots, you will do a great service to the community by using the reCAPTCHA service.
When someone solves the distorted text image to access your email address, he is helping digitize text from old printed books that was not deciphered correctly by the OCR software and required human intervention.
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OCR doesn't always work on text that is older, faded or distorted and the only option is to manually type them into a computer. reCAPTCHA is sending words that cannot be read by computers to the Web in the form of CAPTCHAs for humans to decipher.
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.
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