WP Plugin: reCAPTCHA, stop spam
reCAPTCHA is a free WordPress plugin to stop spam comments and spam feedback. Actually it can be implemented in any php based website.
Most other implementations of CAPTCHAs block visually impaired individuals, who cannot read images of distorted text. reCAPTCHA, on the other hand, has an audio test that allows blind people to freely navigate your site.
Use reCAPTCHA, help digitize books
The words that appear on your captchas are from old books that were scanned but not recognized by the OCR properly. So reCAPTCHA sends them to the participating websites to be read correctly.
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.
WordPress Plugin: http://recaptcha.net/plugins/wordpress/
For PHP sites: http://recaptcha.net/plugins/php/
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