Need a Router/Firewall.. got an old PC?

You can setup a dedicated computer for an industrial-strength and highly-configurable router and/or firewall (free):

Smoothwall : http://www.smoothwall.org/

Router (using webmin): http://ubuntulinuxhowto.blogspot.com/2006/06/setup-your-computer-to-be-router.html

The minimum specifications required for SmoothWall:

Pentium-class Intel-compatible PC running at 200Mhz or higher (hey my friend is giving one away)
32MB of RAM
512MB of disk space.

A more optimal configuration:

500Mhz processor
64MB of RAM
2GB of disk space.

You will also need a CD-ROM drive and at least one network card (typically two, if the WAN interface is Ethernet).

These are just two examples. There are many more available in the Linux community.. such as LinTrack (http://www.lintrack.org/)