Duolingo

Duolingo is a language learning website which offers French, Spanish and German, English (for Spanish speakers), and Portuguese (beta), with Italian and Chinese planned.

It's entirely free and contains no ads.

http://duolingo.com/

The project was created by Luis Von Ahm, creator of Re Captcha (for which he apologizes), a system used to make sure people who make accounts can read and aren't illiterate.
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Lesser known is that it is used to digitize books word by word from image to computer text.

It also spawned the Church of Inglip, numerous memes and a site,

http://www.churchofinglip.com/

 Luis von Ahm then worked on Duolingo, and since its creation on the 30th November 2011, Duolingo has gained exposure and popularity from his talk on TED

http://www.ted.com/talks/luis_von_ahn_massive_scale_online_collaboration.html

and various mentions on Reddit, including an IAMA by Luis von Ahm

http://www.reddit.com/r/IAmA/comments/we7tq/iama_scientist_and_startup_ceo_named_luis_von_ahn/

and a subreddit dedicated to Duolingo, .

http://www.reddit.com/r/duolingo/