Shorewall is a firewall administration package for Linux. I use it heavily. It isn't the easiest, nor does it have a GUI front end. However, it is fairly easy to setup complex firewalling with not a lot of work. It is also easy to setup for simple configurations.
- One-interface systems.
- Three-interface - firewall and dmz router
Tom and the fine folks have release 3.0, and it is working well. I made a conversion guide for the guys at work.