Also known as GFDL and FDL. This license by the Free Software Foundation (FSF) is for documentation accompanying free software. The FDL is controversial, however: Authors may add an invariant section that cannot be modified or removed in derivative works. Because of this, the Debian project has decided to remove all FDL documentation from their "free" section.
The GFDL also has many problems when it comes to wikis. Section 4 has conditions that apply when making modifications.
According to Anthony DeRobertis' comment on the Position Statement about the GNU Free documentation License compiled by Manoj Srivastava , 4B and 4I "seem to prohibit anonymous modifications". In addition to that, it seems that this makes it impossible to use technology such as KeptPages which is designed to forget older history, unless you decide to store the change log separately from the page history.