Attrition's Newbie Area: Newbie-oriented tutorials about stuff like UNIX and system penetration auditing

Def Con: Information about the largest annual hacker convention in the US, including past speeches, video, archives, and updates on the next upcoming show as well as links and other details

Hackerthreads.org: Hacker forum with tutorials about networking, system administration and network utilities

Phrack: Digital hacking magazine

Snake Oil FAQ: Guide to evaluating the quality and fitness for purpose of cryptography tools