SmoothWall. There's a free version that does everything you really need. It lets you use some old computer, that's otherwise useless, to do one of the most important jobs in the house, and do it well.
I wouldn't recommend it, but it'll run fine on a 486 machine with 64 MB RAM and a 4 GB hard drive, I set that hdw up for a home user a while back. A retired Pentium III box makes a smoking hot firewall with SmoothWall.
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