Gamification / Human Computation (Captcha, …)
Example
reCAPTCHA
More than 1 billion people have helped digitise the printed word by using recaptcha
duolingo
duolingo A web site which helps people learn a foreign language (the game), while also providing a translation service (the useful work). People are shown a word or phrase which they do their best to translate; others then vote on the best translation. Duolingo already has 3m users, who use it, on average, for 30 minutes a day.