To quote the WiReD blog:
The idea behind reCAPTCHA is that, as long as we’re all solving these CAPTCHA puzzles, why not throw in some minimal additional data? By adding a second image with an unsolved word from the Internet Archive book scanning project, ReCAPTCHA allows users to channel their CAPTCHA solving skills into real world benefits.
To me, this is an innovative idea of putting the world to work. I wonder how many more companies are employing this approach?