These organizations range from individuals to non-profit organizations to businesses. Some of the pages contain information on not only how to secure your systems but how to break into systems as well. Here is a sample of some of the better security related pages I have found. All the organizations listed here handle incident response, provide security suggestions, guidelines, or checklists for admins to improve the security of their systems, or publish security advisories. Many also have tools that admins can download to check or improve their sites security.
Some businesses have web pages with various computer security infomation available as well.
There are also a number of businesses that provide security services and solutions. These links only list those businesses that make information generally available on the Internet. Do not consider this list an endorsement of these businesses or there products. The sites listed here are considered 'underground' sites by many because contain information on how to break into systems. This information ranges from papers describing general concepts to source code to exploit various security holes to gain access as a user or even root.