Have a look at these sites especially the guides and tutorials. You can use their links to find other sites and so on.
- http://www.antionline.com: Has excellent tutorials section in the forums.
- http://www.hackthissite.org: Lots of challenges with tutorials and lectures
- http://www.tgs-security.com: Has forums, tutorials, tools
- http://securityfocus.com/library: Has a lot of very good articles on how to secure computer systems
- Open Source Security Testing Methodology Manual: It is is a peer-reviewed methodology for performing security tests and metrics.
- Security Forums: Has really good discussions on security related topics.
More to be added soon (As and when I go through my bookmark collection to find them)
– Suramya