Advanced bash scripting guide (4,255)
A 400+ page document that goes into great detail on the subject of scripting in the bash shell (which can be installed on OS X).
BigAdmin System Administration Portal (2,940)
A collection of various UNIX hints with explanations. This is actually at Sun and pretty Sun oriented, nevertheless there is some really nice stuff here. Also be sure and check out the "Resources", and "ShellMe", and "How To / Tutorials" links at the site.
CMS Comparo Page (2,207)
An amazing page that lets you not only see but also administer 10 different CMS (Content Management Systems), including Geeklog, PostNuke, PHPNuke, etc.
Every hour, the sites are all restored to "new install" status, so you can do whatever you like as admin for an entire hour...
Compiling Source Code How-To (2,761)
A great overview of the process involved in compiling UNIX apps. Not specific to OS X, but very applicable.
Extropia's online UNIX course (2,178)
An online UNIX course targeted at teaching web developers about UNIX. Much of the knowledge is general and very applicable to OS X.
freshmeat OS X page (3,773)
Many OS X users who have come from (or still use) Unix variants such as Linux or BSD will have heard of freshmeat.net - well, freshmeat finally has a section devoted to Mac OS X software. Software authors are encouraged to submit their software to freshmeat, or software users to "adopt" a project and keep its record updated.
geek-girl UNIX guide (4,275)
A very comprehensive collection of UNIX-related web resources, including BSD (the UNIX base in OS X)
Hands-on How To (4,384)
Although targeted at Linux users, this site has some excellent tutorials on the command line, Samba file sharing, CGI, DNS setup, and other topics of interest. Very well written and easy to follow, although some of the topics would qualify as 'advanced'!
Installing Qt on Darwin (2,225)
Sashoto has posted his notes of getting Qt installed on Darwin. Some of them are well known issues, but at least they are well organized and I found them very helpful.e
Qt is a cross-platform C++ GUI framework for developers, from Trolltech: