Open Source Network Gateway | Untangle
Basically a network security appliance distro, based on Debian I believe. It's got all sorts of free plug-ins for network security, like web filter, spam, virus, spyware, phishing, etc. plus it's also a VPN server, firewall, and router. Check it out:
It's got a virtual rack (like Reason, but for network nerds ) that you can drag stuff onto for added functionality. Pretty configurable too.
I've already pissed off my wife because I blocked MySpace yesterday just to see how it works.
I'm running it headless (configuring through web browser) on an older HP D530 tower (2.6Ghz P4, 1GB RAM) and it's fine, though of course we're not running a lot of traffic through it. Something like this:
with a RAID 1 config and dual NICs (plus a 3rd NIC for DMZ) would be nice.