Squid is an open source proxy server. It is full-featured Web proxy cache, designed to run on Unix (linux/BSD/Solaris etc) systems, free and open-source software.
As Linux admin most of the time people ask me about Squid proxy license. Squid is copyrighted by the University of California San Diego. Squid is licensed under the terms of the GNU General Public License.

For more inforamation please visit:
GNU General Public License and GNU project.
Offical squid website