Oxford Univ. OS X Intro (1,430)
A very well-written introduction to OS X which covers things like permissions, installing software, and backing up data.
Packages explained (1,497)
A good overview of the packaging system used in OS X (and available since OS 9). It explains how things are bundled together to disappear from view for most users.
Pixits defaults page (1,315)
Page dedicated to the defaults commands in MacOS X. The site also hosts Pixit's MacOS X applications
ResExcellence OS X Snapshots Page (1,991)
A collection of 300+ OS X desktop screenshots, complete with explanations as to what's on the screen. Serves as a great resource for not only the look of your desktop, but finding cool OS X applications as well.
The MacHelpDesk.com (1,906)
Tips, reviews, tutorials and editorials from a member of the Apple Cosultants Network.
The X Lab - Troubleshooting Mac® OS X (1,699)
The X Lab is dedicated to providing outstanding troubleshooting information and services to users of Mac® OS X. The X Lab was founded by "Dr. Smoke," the screen name of an Apple Discussions Helper and author of the e-book "Troubleshooting Mac OS X."
Troubleshooting Mac® OS X, Second Edition (2,639)
The X Lab™ has published "Troubleshooting Mac® OS X, Second Edition," a major update to their e-book dedicated to preventing or resolving common problems with Mac OS X. The Second Edition covers both Mac OS X 10.3 Panther™ and Mac OS X 10.2 Jaguar. The book is extensively hyperlinked and cross-referenced, both within the book itself and to some of the best Mac OS X troubleshooting information on the Internet.
What is Mac OS X? (1,163)
A detailed look at OS X at the deepest levels. Not necessarily for "GUI only" users, but if you're interested in what happens behind the scenes, this is an interesting read.