GFDL

The GNU Free Documentation License (GNU FDL or simply GFDL) is a copyleft license for free content, designed by the Free Software Foundation (FSF) for the GNU project. It is the counterpart to the GNU GPL that gives readers the same rights to copy, redistribute and modify a work and requires all copies and derivatives to be available under the same license. Copies may also be sold commercially, but if produced in larger quantities (greater than 100) then the original document or source code must be made available to the work's recipient.

The license was designed for manuals, textbooks, other reference and instructional materials, and documentation which often accompanies GPL software. However, it can be used for any text-based work, regardless of subject matter.