The last time I played around with IPTables was about 6 - 7 years back. I have been working with a Linux box again from the last few months and I am absolutely thrilled about it. Nevertheless, I have also been meaning to set up a simple IPTables firewall on my machine.
Since I had forgotten all my IPTables concepts, I decided to hunt the Internet for some good articles. I found some really nice resources to refresh my memory as well as learn new things.
Here is a nice 3 part video series from Linux Journal.
Here is a nice picture which explains how a packet is routed through various chains in the ip tables. If you are looking for a quick refresher, this might help you out.
If you want more details, here is a tutorial, and yet another tutorial, and Netfilter's excellent documentation on IPTables.
So now there is no excuse for not setting up a little firewall on your Linux box.