GNU General Public License
The GNU General Public License, or GPL, is a license for free software. It is under copyright and owned/maintained by the Free Software Foundation. The license is designed to ensure that any program using it, or work derived from such a program, will continue to be freely available at no cost.
See this page at gnu.org for an english version of the GPL.
See Richard M. Stallman: Selling free software for background.
See the Wikipedia entry on the GPL for more information.