Some links for software firewalls to create dedicated firewall PC's:
2020: TODO: Look at http://www.orangepi.org/ running LEDE or OpenWRT rather than hacking a commercial router.
2016 - http://vyos.net/wiki/Main_Page - fork of Vyatta - can do some crazy stuff - apparently vyatta got swallowed by Brocate - http://vyos.net/wiki/Vyatta
2015 - some new names on this thread from a couple years ago http://community.spiceworks.com/topic/350102-smoothwall-getting-long-in-the-tooth:
untangle.com
endian.com
---------
Obligatory slashdot article:
http://linux.slashdot.org/article.pl?sid=05/08/06/1933229
pfSense: Wow, pfSense packages are awesome. setup of squid as transparent proxy was only a few clicks. pfSense was a fork from monowall.
OPNsense: when monowall shut down, they referred people to OPNsense.
Classic names: smoothwall, monowall, pfsense
Tags: OSS, GPL, firewall, router, NAT