The NSA is, of course, the United States National Security Agency. It’s their job to a) keep the nations computers secure; b) find out how to break everyone else’s (ah, the dichotomy of national intelligence!). Thus, some of the best computer security minds are at the NSA – it was the NSA that brought us SELinux (and still does!).
They also have a vast array of security guides available for download. These include guides on securing Solaris 8 and 9, MacOS X 10.3 and 10.4, Windows 2000, Windows XP, Windows Server 2003, Windows Vista, and more.
Use a security guide next time you secure a box – and not necessarily just one. Do some research. Implement all of the security you can on all of your boxes – even if it is not on the Internet. One day, someone might just crack through – then all of your internal systems will be at risk. Each system should be able to withstand an assault without falling.