Smoothwall Express in a Linux distribution designed specifically to build a PC as a firewall.
From the download page at: http://www.smoothwall.org/download/
“Express 3.1 RC5 is the fifth Release Candidate for the 3.1 version of Smoothwall's Free, open source GPL network firewall.
Like Express 3.0, two architectures are available; one for Intel® i386 and one for 64bit processors.
In addition, three release types are available:
- Standard - This is the normal release type for users.
- Developer - This is a release targeted at people who want to work on Smoothwall Express.
- Offroad - A cut down version for people who want to ensure their system's hardware is compatible with Smoothwall Express.”