I found an interesting article today explaining how the reCAPTCHA system works and how it helps in the effort to digitize books. I think now I might possibly consider all those reCAPTCHAs I come across a bit less of an annoyance.
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.
Read more at: What is reCAPTCHA?