Robert Graham's Infosec publications : includes a lot of interesting information, including a list of port seen from a security point of view.
NDISbench : test your network intrusion detection system.
Security Focus : the best security site around.
The Common Intrusion Detection Framework homepage.
Shadow's Homepage - the US Navy Intrusion detection system.
Snort - a lightweight network intrusion detection system.
Nessus - the freeware, open source vulnerability scanner.
Buffer Overflow Primer by DilDog.
50 tools for the hacker, from the insecure.org site.
ntobjective's free tools - IP packet creator, forensic tools, log cleaner...
cotse - information resources for the IT professional.
Counterpane's research pages.