GNU Free Documentation License
The GNU Free Documentation License (GFDL, GNU FDL) is a copyleft free content license that is frequently used for wiki the contents of wiki websites. It was designed by the Free Software Foundation especially for documentation of software and is one of the oldest free content licenses. The specialisation on software documentation brings about some problems when used for other sorts of content; see article GFDL Concerns for more information. The most notable user of the GFDL, the Wikipedia and its sister projects of the Wikimedia Foundation, moved from licensing their contents under GFDL to a double licensing scheme with the comparable Creative Commons Attribution-Share Alike license, when the GFDL grantedd the one-time chance to migrate content to this Creative Commons license in 2009.
Here on WikiIndex you can find wikis with contents under this license listed in the category Wiki GNU Free Documentation License.