GNU General Public License
The GNU GPL is, according to various statistics, probably the most used free software license. It was also the first license to implement the concept of copyleft, guaranteeing that "GPL'ed" free software cannot become, or take part in, non-free software.
Although the GPL is primarily intended for software programs, it is worded so as to apply to many different kinds of works. The main condition for the GPL to be applicable to a type of work is that it admits the notion of a preferred form of a work for making modifications to it (be it source code in a computer language, music score notation, digital graphics under a format retaining structure, etc.). For example, there are many occurences of text or graphics released under the GPL.
<a rel="license" href="http://creativecommons.org/licenses/by-nc-sa/3.0/"><img alt="Creative Commons License" style="border-width:0" src="" /></a>
This work is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-sa/3.0/">Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License</a>.