Attrition's Newbie Area:Newbie-oriented tutorials about stuff like UNIX and system penetration auditing.
Hackerthreads.org:Tutorials about networking, system administration and network utilities.
Hackers World Portal: Links to hacking history, cryptology, censorship, international hacking groups, news, publications, security, weekly features.
How to Become a Hacker: A short essay on becoming an "old-guard" hacker (NOT a cracker or phreaker).
How to become a Hacker: ESR's paper on how to become a Hacker.
Linux Documentation Project: Compilation of tutorials and HOWTOs for the linux operating system.
The Happy Hacker: Home of the Guides to (Mostly) Harmless Hacking. Focus is mainly on Windows 95 and 98. Some mention of Windows NT.
The New Hacker's Dictionary: Hacking jargon including information on writing style, hacker terms, and lamer-speak.
^Rob's WerLD: Contains information about viruses and trojans and their fixes. For newbies and advanced.