http://www.insecure.org/nmap The Nmap home page. There is a lot of general information about network security here, as well as official and user-generated documentation.
http://www.rootkit.nl/projects/rootkit_hunter.html Analyze whether an attacker left behind tools to wreak more havoc on your system.
http://www.securityfocus.com Keep up with security issues on all platforms.
http://www.linuxsecurity.com Focus on Linux security problems and solutions.