I've just seen a load of "give me your iptables lists"-style questions and thought that the following might be useful.
There used to be a file called Projectfiles rc.firewall - it later moved (briefly) to lfw.sourceforge.net but it seems to have now died. It's a plain bash script that sets up a fantastic iptables config and which you can use on servers, clients and routers without hassle.
It has a lovely, simple, "variable" config and the default (i.e. without changing a thing in the file) is a perfect single-computer firewall. Just by adding a list of internal interfaces to one variable you get a fully-protected NAT gateway.
It's perfect for Slackware, especially, as it just goes into /etc/rc.d/ where it will automatically be executed at startup. It's very customisable and creates some very strong rules without needing to know a single iptables command. Additionally, it's requirements are absolutely minimal and it has no dependencies on anything (except bash, of course).
When I found out that it was no longer easily available on the net I went through all my backups and found the documentation and several versions of the scripts, which I have posted on my blog here:
I thought that a few people might find this useful. I've heard several people on these forums recommend this particular script and it would be a shame to lose it forever.