reCAPTCHA from Carnegie Mellon University

"Carnegie Mellon researchers have launched a new service that will not only protect e-mail addresses on the web from spambots, but also help digitize a backlog of old books, magazines, and newspapers so that they can eventually be computer searchable. The service, called reCAPTCHA, hopes to use the eyeballs of millions of Internet users to identify thousands of words for the Internet Archive"