From their "What is reCaptcha?" page:
About 60 million CAPTCHAs are solved by humans around the world every day. In each case, roughly ten seconds of human time are being spent. Individually, that's not a lot of time, but in aggregate these little puzzles consume more than 150,000 hours of work each day.Very clever and very fun.
What if we could make positive use of this human effort? reCAPTCHA does exactly that by channeling the effort spent solving CAPTCHAs online into "reading" books.
reCAPTCHA improves the process of digitizing books by sending words that cannot be read by computers to the Web in the form of CAPTCHAs for humans to decipher ... Each word that cannot be read correctly by OCR is placed on an image and used as a CAPTCHA.
For more on Luis Von Ahn's work, see the discussion and links to talks and papers in my earlier post, "Human computation and playing games".
[reCaptcha found via O'Reilly Radar]