Free Software
Free as in freedom (i.e. freedom to use, read, share, modify etc.)
The GNU GPL is a free software license used by many free software projects.
See http://www.gnu.org/ (the Free Software Foundation)
See Also
Here's a great article about free software: http://moglen.law.columbia.edu/publications/anarchism.html
How To Become A Hacker: http://www.catb.org/~esr/faqs/hacker-howto.html
How the hacker culture functions: http://catb.org/~esr/writings/homesteading/