Wikipedia was launched on January 15, 2001 (English only), as a complement to expert wiki Nupedia (closed in 2003), by Jimmy Wales and Larry Sanger. The name is a play on wiki and encyclopedia. Wikipedia is run by the Wikimedia Foundation, where Jimmy Wales still resides, and runs on its own open source wiki software called MediaWiki.
Wikipedia is available in more than 250 languages and millions of articles. The idea is that the users contribute to the wiki where they have knowledge, thus always keeping it up to date. There is of course the problem of credibility, why most academic institutions won’t accept Wikipedia as a resource, but the rise of the wiki has certainly made it the encyclopedia of choice for lots of people. According to Alexa, Wikipedia is currently in the top 15 sites of the world.
The English edition of Wikipedia is by far the largest one, with more than 1.5 million articles. German and French versions are around 500,000, while small countries (in comparison) such as Portugal or Sweden have around 200,000 articles in their language.
Wikipedia content is open source, just as the MediaWiki software.