SecuriTeam
Beyond Security will help you expose your security holes and will show you what the bad guys already know about your hosts and network.
SecurityFocus
SecurityFocus is the most comprehensive and trusted source of security information on the Internet.
Computer Security vulnerabilities
Computer security and information security: advisories, exploits and vulnerabilities database, articles and security news.
Black Hat
Black Hat Conventions put you face to face with people on the cutting edge of network security, and with no vendor pitches!
SANS
Computer security training, certification and free resources. We specialize in computer/network security, digital forensics, application security and IT audit.
The Metasploit Project
The Metasploit Framework is the open source penetration testing framework with the world's largest database of public, tested exploits.