SmoothWall is a specialised Linux distribution designed for making an old obsolete PC into a fully functional NAT router and firewall, with an optional Squid web cache, VPN, port forwarding / DMZ, Snort Intrusion Detection System etcetera. After installation all configuration is done via a web interface, which can be used amongst many other things to bring the link up and down and even update modem firmware and itself remotely. In short, it's really cool and a definite candidate for "what do I do with that old PC?"
A Free GPL version and corporate supported version are available :