Firewalls 101: How stuff works DOT com. Great for establishing a foundation for understanding the firewall.
Firewall FAQ great for newbies and rusty IT Security test takers from the undisputed champions on Infosyssec who continue to hack the hackers.
Exhaustive Firewall Links from CERIAS.
Another great article by Cisco. White Paper on Cisco's PIX firewall and Stateful Firewall Security.
ISAserver.org: The No.1 unofficial ISA Server 2000
& 2004 resource site – This feed offers news, articles, tutorials
and reviews for ISA administrators. —> ISA SERVER/FIREWALL FEED
Dr. Tom Shinder's ISA Server Firewall Blog. Not updated very often, but good info on ISA firewalls.
Home PC Firewall Guide. Comprensive site about configuring firewalls.