On http://www.cromwell-intl.com/security/ you will find a number of articles related to Computer System and Network security. It covers chryptography, privacy tools, computer forensics, authentication, intrusion detection and network monitoring.
GRC Shields Up! tests your firewall from the outside and reports what you are exposing to the Internet.
At CERT, Computer Emergency Response Team, you will find up to date information about security related issues like newly discovered vulnerabilities. Stay updated!