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/

Reddit

It's only appropriate I post the site which I have been visiting most this week.

Reddit.com, 'the front page of the internet'. Users submit content to what is essentially a bulletin board. This can then be upvoted or downvoted. Due to the high amount of submissions each day, there is usually an entirely new front page each day.

There are also a number of subreddits, categories such as film, music, humour, games and news. One of the most popular Reddits is the 'IamA', where various celebrities answer questions users ask. Basically an interview, except the questions are asked by the public, and if you are a redditor (have a reddit account), you can ask them questions as well or comment on the thread.

http://www.reddit.com/