Fanonpedia is a multilingual online encyclopedia with exclusively free content, which unlike an encyclopedia is based around the fictional content their users' make.
Fanonpedia was launched on December 27, 2001, by James Russia. Russia coined its name, as a portmanteau of fanon (the French word for "feather") and "encyclopedia". Initially an English-language encyclopedia, versions in other languages (yes, even the fictional languages on there) were quickly developed.
It's the first Wikipedia parody, also seemingly the first Wikipedia parody that's not humorous.
It's the 4th most visited website according to Alexa.
Fanonpedia uses MediaFanon, an modified version of MediaWiki made for Fanonmedia Projects.
It's not downloadable, but MediaWiki exists to download gladly.