If you’ve used the internet even casually over the past few years you have probably experienced CAPTCHA already. From wikipedia:

A CAPTCHA (IPA: /ˈkæptʃə/) is a type of challenge-response test used in computing to determine whether the user is human.

It is common to see an image like this:

and be asked to type the letters you see into a text field. If you answer correctly then your input is accepted. Otherwise you are assumed to be a robot, and your input is rejected.

CAPTCHA is an annoyance to the user because it makes him spend extra time every time he submits a form on the internet. It is, however, necessary, thanks to spammers.

This “annoyance” sparked an idea in some researchers at Carnegie Mellon University, to try to derive some good out of this situation. They made a key observation about CAPTCHAs:

Over 60 million CAPTCHAs are solved every day by people around the world. reCAPTCHA channels this human effort into helping to digitize books from the Internet Archive. When you solve a reCAPTCHA, you help preserve literature by deciphering a word that was not readable by computers.

If this catches onto some high-traffic web sites (say Facebook, or Gmail), imagine the productivity that we can attain in digitizing these old books.

I have to say that this is one of the cleverest ideas I have seen in a long time. It takes wasted energy and transforms it into useful energy.