Apple has released a comprehensive Security Configuration guide for OS X 10.5 in PDF. Designed for experienced users the "guide provides instructions and recommendations for securing Mac OS X version 10.5 or later, and for maintaining a secure computer." Among a variety of topics it covers the OS X security architecture, installing OS X, protecting hardware, securing global system settings and account settings, as well as providing some OS X Server specific information.
The guide is aimed at the "experienced Mac OS X user, [who should] be familiar with the Mac OS X user interface, and have some experience using the Terminal application's command-line interface." In other words, you need to know what you're doing before you start digging around in things which "could cause serious adverse effects on the computer and its security." Nonetheless, if you're a power user or system admin who wants to get a handle on securing OS X this guide is definitely worth the download (pdf link)
[via Daring Fireball]