Very entertaining presentation by Louis Von Ahn, of how the Captcha registration process has been put to effect social good:

“After re-purposing CAPTCHA so each human-typed response helps digitize books, Luis von Ahn wondered how else to use small contributions by many on the Internet for greater good. At TEDxCMU, he shares how his ambitious new project, Duolingo, will help millions learn a new language while translating the Web quickly and accurately — all for free.”