SmoothWall is a Linux distribution that converts an old or redundant PC into a hardened Internet firewall device. It's hardware requirements are modest, a home or small office network can be handled by a 486 with minimal memory. The PC only needs a keyboard, monitor and CD drive for the initial installation. After that, all setup and administration is done via a web browser from any other computer on the local network. The Internet connection can be ethernet through an existing modem or router or one of the many internal modem, ISDN or ADSL cards that are supported.
A SmoothWall Express box can also act as a wireless access point and can be set up so that wireless users have more restricted access, to reduce the harm that a wireless intruder could cause. There is also full support for a DMZ (De-Militarised Zone) to make servers or other computers accessible from the Internet and give them reduced access to the local network. All of this is handled by the web interface that sets up various zones for different uses. Proxy servers for web and mail are included, along with email virus scanning and filtering. Traffic graphs show when and for what your network is used most.
SmoothWall Express is the free and open source version of this appliance and comes with community support. There is also a fully supported commercial version available from http://www.smoothwall.ltd.uk.
WARNING: SmoothWall Express is not to be installed alongside anything else as it takes over the entire computer. Installing this will ERASE your hard drive unless you intend to install on it's own drive.
A good use for that old PC you no longer use, and more powerful than an off-the-shelf router.