NetBSD/i386 Firewall is a free firewall solution for people with
a permanent Internet connection. This includes most users of cable
or ADSL services, but also businesses with leased lines. It requires
a PC (any old one will do) and two ethernet cards. Currently only
for folks with a fixed IP address, but the site maintainer is
working on a version for those of us with dynamic IP addresses.
The main target is to make installation of an rock-solid and very
secure firewall as easy as possible.

For more details see: http://www.dubbele.com/