Danila Narcisi

A "captcha" is a computer program that can tell whether its user is a human or a computer hence stopping spammers from gaining access to an email address through automated programs.

reCAPTCHA is a free "captcha" service that helps to digitise books, newspapers and old time radio shows.
In their own words: "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."

So, if you want to help them to archive human knowledge and to make information more accessible to the world, add reCAPTCHA to your website! I did it (see the "contacts" page), and it works.
To learn more about reCAPTCHA, click here.