Intro to OS X UNIX (4,199)
MacInstruct has posted a fairly interesting interactive primer on using the Terminal application in OS X. This tutorial allows you to interact with a synthetic Terminal application to get a feel for how the commands work and what they do.
If you're very new to UNIX and OS X, this is a good overview of the capabilities of the Terminal program which gives you direct access to OS X's UNIX engine.
IT Enquirer's OS X Guide (1,423)
A very thorough tops-to-bottom look at OS X, starting with preparation, installation, and a look at the system preferences panels.
J. Blake Harris' OS X page (1,736)
A page with some cool icons for the iBook and for the Fire IM manager, as well as a replacement poof (no instructions; see them elsewhere on osxhints), and some new "favorites" icons (also no instructions; find them on xicons.com).