The GNU license
The GNU Free Documentation License, or GFDL produced by the Free Software Foundation concerns authors' rights. It aims at protecting free works. Anyone can choose to use it, it is the license under which work is published in Wikipdia.
With this license, any work is made available for free use : it can be copied or redistributed, modified, and even commercial purposes.
It's based on the principle of the copyleft GPL used for free (open source) softwares.