SmashTheStack is a place for like minded individuals to participate in wargames to test their knowledge and to learn more about hacking in a safe manner. It's community driven and is worth a visit. It even offers a free shell service.
   http://www.smashthestack.org/

Deep in the recesses of any computer, innumerable processes are running silently in the background. Some hog system resources, drastically slowing computers down; others harbour spyware and Trojans violating privacy and giving hackers free reign on compromised systems. Processlibrary.com is a free online resource for anyone who immediately wants to know the exact nature and purpose of every single process. This popular easy-to-use service is provided free-of-charge by Uniblue Systems.
   http://www.processlibrary.com/

MakeSecure.com is an information security web site, providing security information to the community. Includes Tools, Links, News, Advisories and online remote security lab.
   http://www.makesecure.com/

SecurityFocus.com is designed to facilitate discussion on security related topics, create security awareness, and to provide one of the Internet's largest and most comprehensive database of security knowledge and resources to the public.
   http://www.securityfocus.com/

Infosec Writers are online publishers of information security papers and projects, working with established and unestablished writers in the industry. The overall community has the opportunity to rate submissions and partake in related forum discussions.
   http://www.infosecwriters.com/

Founded in June of 2005 as the brainchild of Pedram Amini, the Open Reverse Code Engineering community was created to foster a shared learning environment among researchers interested in the field of reverse engineering. Heavily modeled on the architecture of Greg Hoglund's rootkit.com, OpenRCE aims to serve as a centralized resource for reverse engineers (currently heavily win32/security/malcode biased) by hosting files, blogs, forums articles and more.
   http://www.openrce.org/

A large compilation of security links including Security News sites, Security Advisory sites, Security Scanners, Programming sites, Anti-virus, Phreaking sites, E-Zine sites, and more. All sites are rated for quality.
   http://www.startplaza.nu/

SecureMac.com contains Macintosh security news, reviews, advisories, and security tools for Mac OS and OS X.
   http://www.securemac.com/

Windows security site which provides Windows security news, articles, tutorials, software listings and reviews for information security professionals covering topics such as firewalls, viruses, intrusion detection and other security topics.
   http://www.windowsecurity.com/

Freak's Mac Archive is the de facto standard for Macintosh hacking tools and information.
   http://freaky.staticusers.net/

eBCVG is a security portal dedicated to providing security professionals with the knowledge and resources to help protect all of their data. They provide daily updated news, information on the latest viruses, security articles and more.
   http://www.ebcvg.com/

Help Net Security has been online since 1998. Initially conceived as a download archive, the site has grown into a daily updated security related news site with lots of additional content.
   http://net-security.org/

Founded in 1994, the Hacker's Choice (THC) is a group of security engineers that through the years have released several papers and applications covering telecommunication, credit card, Unix and NT security.
   http://www.thc.org/

Attrition is a computer security site dedicated to the collection, disemination and distribution of information about the industry for anyone interested in the subject. They maintain one of the largest catalogs of security advisories, cryptography, text files, and denial of service attack information.
   http://www.attrition.org/

Security Tracker is a site devoted to tracking security vulnerabilities, and nothing else.
   http://www.securitytracker.com/

An international Dutch view upon the networking and security world. DSInet.org provides its visitors with information, files, tools, news items, columns, opinions and an editorial view upon the genre in general.
   http://www.dsinet.org/

The CERT Coordination Center is part of the Survivable Systems Initiative at the Software Engineering Institute, a federally funded research and development center at Carnegie Mellon University. Originally started by DARPA in 1988, their primary focus is on incident response.
   http://www.cert.org/

Not updated often, but it's a decent listing of default TCP/UDP ports in use.
   http://www.neohapsis.com/neolabs/neo-ports/

Computer Operations, Audit, and Security Technology is a multiple project, multiple investigator laboratory in computer security research and functions with close ties to researchers and engineers in major companies and government agencies.
   http://www.cerias.purdue.edu/coast/

USSR has many homegrown exploits and advisories, encryption resources, and much more.
   http://www.ussrback.com/

Zone-H is a large computer crime archive and resource for security news, advisories, and tools for IT professionals.
   http://www.zone-h.org/

Phreak.org also offers a wide variety of files, links, and data in relation to accessing phone systems.
   http://www.phreak.org/

Hakin9 is a printed magazine available in a number of Central and Western Europe an languages (including English). The overall technical quality of the articles is extremely high, and (being a bi-monthly publication) the content is quite a b it ahead of what you find in books. The magazine is available both in PDF and ha rdcopy form. The price at the time of writing was 20 euros for a yearly softcopy subscription / 30 for a yearly hardcopy subscription.
   http://www.hakin9.org/

Phrack was an electronic magazine with a 20-year history of detailing insecurities in systems. The first known PUBLIC description of buffer-overflows ("Smashing the stack for fun and profit", by Aleph One) appeared here.
   http://www.phrack.org/