The encyclopedia project of the Wikimedia Foundation.
You can link to their pages here using [[Wikipedia:Page]] or [[DeWikipedia:Seite]] for the german Wikipedia.
Wikipedia exists in almost every language of the world.
They have the according IRC channels for each project on the Freenode IRC network.
One of the lead developers is Brion Vibber.
The wiki engine of Wikipedia is called Mediawiki and the organization that runs it is the Wikimedia Foundation.
We are also using the Mediawiki engine here on S23 Wiki.