reCAPTCHA is a free CAPTCHA service that uses text that OCR scanning technology couldn’t translate as the random text:
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. (reCAPTCHA Website)
Truly, we live in the future.
I’m now using this system for user registrations (which are required to leave comments). So go register, and help read books!