If you consider yourself as Linux administrator this post might be useful for you. NSA published some nice papers with hardening Linux servers recommendations. NSA - National Security Agency - organization that is trusted. Latest version of 200 hundred pages guide in PDF for securing configuration of RHEL5. A very detailed manual, explaining general principles of secure system, including all necessary system changes, service configuration, network security, firewall, etc..
A must have thing for every admin. If you are Debian or Ubuntu admin it still worth reading, since principles stays the same and there is no much real difference in security between RHEL and Ubuntu.
Besides RHEL, there are many guides for other OSes as well
- Secure configuration of Mac OS X 10.6 Snow Leopard (October 2010)
- Secure configuration of Mac OS X 10.5 Leopard (March 2009)
- Using of Software Restriction Policies (SRP) in Windows (March 2011)
- Security highlights in Windows 7 (May 2011)
- Windows Vista security guide (March 2009)