There is a lot of resources available about hacking and security, here are my favorites.
Hackademics (fr): french forums about hacking techniques.
SecurityTube: incredible pool of hacking videos, security courses and certifications.
Zenk-Security: online challenges and tutoriels repository.
Les Tutos de Nico (fr): french blog about security.
Hacker Web Directory: security resources list (many broken links unfortunately).
Penetration Testing Lab: great blog about pentest techniques.
Pentest Geek: another nice blog with nice archives
OWASP: “The Open Web Application Security Project worldwide not-for-profit charitable organization focused on improving the security of software.”
PTES Technical Guidelines: penetration testing guidelines.
BlackHat Library: nice wiki with vulneabilities explanations (seems to be down).
5m4ru (pl): polish hacking articles with few videos.
InfoSec Institute: a lot of great articles about security and hacking.
DEF CON: annual meeting in Las Vegas with the best hackers and security experts in the world.
SecTools: pool of network security tools
Les bases du hacking (fr): introduction to basic hacking techniques. You don’t need any experience to understand the steps presented in this book.
The Web Application Hackers Handbook (2nd): “This fully updated edition contains the very lastest attack techniques and coutermeasures, showing you how to break into today’s complex and highly functional applications.”
Metasploit: The Penetration Tester’s Guide: Metasploit is probably the most popular framework used by professional pentester.
Ethical Hacking and Penetration Testing Guide: introduction and guidelines to understand offensive security.
to be continued…