All major projects of the Wikimedia Foundation are collaboratively developed by its users. For this purpose these projects are using a wiki software called MediaWiki. The content is free under the GNU Free Documentation License, meaning that it may be freely used, freely edited, freely copied and freely redistributed subject to the restrictions of that license.
Wikipedia
Wikipedia is a multilingual project to build free encyclopedias in all languages of the world. The project startet in january 2001 and has nearly reached 1.000.000 articles in about 140 languages. The largest Wikipedia is in English language, followed by German, Japanese and French.