This is really cool. reCAPTCHA is a free CAPTCHA service that asks the user to type two words. They know the first word, but the second comes from a failed OCR scan. If you get the first word right, reCAPTCHA assumes you’re human, and they store your answer to the second word…they even run it by a few other humans to raise the confidence. Not only are you authenticated, you’re transcribing. According to reCAPTCHA, “in aggregate these little puzzles consume more than 150,000 hours of work each day”.
[For the record, I don’t know anyone at reCAPTCHA. I found them while reading up on CAPTCHAs for work.]