Have you had to decipher "words" at the bottom of a page before you could submit a form online?
The "reCAPTCHA" program, designed by the Computer Science Department at Carnegie Mellon, helps computers translate words that cannot be read through the optical character recognition program (OCR) used for digitizing books.
If a word can't be read through OCR it goes into the CAPTCHA list to be sent out in a pair with a known word. The computer assumes if you get the known word right, the other one is solved correctly, too. Through multiple human responses to an unknown CAPTCHA, the computer accumulates data to determine the most probable characters. Wow!
A related service uses CAPTCHAs to protect your email address if you wish to post it on the web. Mailhide generates an HTML code that will force a potential user to solve a CAPTCHA puzzle to get the email address.