Complements

The GNU operating system, popularly know through its usage in GNU/Linux (often called just Linux), is a completely Free implementation of a Unix-like operating system.
It's Free in a sense that it gives the users the right to freely use, study, modify and distribute the software, as opposed to the much weaker "use without charge" definition of "free".

Learn more at http://gnu.org or at the Free Software Foundation website, http://fsf.org