GNU Free Documentation License
The Free Software Foundation's copyleft licence for manuals, best known for being adopted by Wikipedia and then abandoned by it.
The FSF released the GFDL in 2000 so that manuals could enjoy the same freedom as the programs they describe. One quirk caused a fight: "invariant sections" that no downstream editor may change or remove. The FSF used them for political essays in its manuals, and in 2006 Debian voted that documents containing them don't count as free.
It was still the obvious choice for Wikipedia in 2001. But it was written for printed books — including a requirement to ship the full licence text — and fitted poorly with articles edited by thousands of people. In 2008 the FSF added a time-limited migration clause to version 1.3, and in 2009 Wikipedia moved to Creative Commons BY-SA.