GPL
The GNU General Public License is the "copyleft" license used by most of the applications developed by the Free Software Foundation (FSF). Software released using this license is free according to the DFSG

Although it is well-understood for traditional C/Unix programs, fewer people agree on exactly what its implications are when used in a typical CL environment. At least, I don't. Debate welcome

For more details see its page on the GNU project web site