Have your firewall running but not sure it's working?  Try these Firewall Test Resources

If you are interested in Firewalls, you might also want to look into some great Internet Connection Sharing Info

Distributed Intrusion Detection System - Compiles firewall logs from people and compiles hacking attempt statistics.

Port Assignments for All Well-known and Registered Ports http://www.iana.org/assignments/port-numbers

Port Assignments for Well Known Ports http://en.wikipedia.org/wiki/List_of_well-known_ports_(computing)

SNORT for Windows, Linux, OSX and more

Snort is a lightweight network intrusion detection system, capable of performing  real-time  traffic analysis and packet logging on IP networks. 

TUCOWS:  Don't forget to check TUCOWS list of software for firewall tools.  They have many!