clipped from www.wired.com

But reCaptcha has an even sneakier — and more delightful — purpose. The words are pulled from the book-scanning project of the Internet Archive

One of the two words in the test is the control word: The gatekeeper computer knows what it should be, so it's there to make sure the puzzle-solver is indeed human. But the other word is there for a different reason. The Archive's scanners are good, but some of the words are too smudgy for the software to decipher. The game takes the image of each smudgy word and puts it into reCaptcha. Each time someone completes a reCaptcha puzzle, they'll be doing a tiny bit of work — translating that difficult image into text

Roughly 50 million Captchas are solved each day. If von Ahn can acquire just a fifth of those users, he'll have a stunning 30,000 daily man-hours of work at his disposal. It would constitute the world's fastest and most accurate character-recognition computer, processing 10 million words a day.