This page contains a variety of links to World of Warcraft related resources that I have personally found useful.
The Undermine Journal
The Undermine Journal (TUJ) is a great source of gold making information, including best buys and email notifications of bargains on your server.
Wowuction is another auction house datamining site. Information here is compiled and presented differently from TUJ and is preferable in some areas. There is also a wowuction addon to allow import of prices into in game tooltips.
Home of the excellent Trade Skill Master addon and source of some great gold making information.
Macro and Interface Related
Wowinterface is the home of some of the best addons available in World of Warcraft. It has an active and helpful forum community.
Home of literally thousands of addons for World of Warcraft.
An excellent macro extender (used to take macros beyond the 255 character limit). Does a lot more than merely allow macro extension and is required for many sophisticated macro used on this site and others.
This is a complete UI replacement and not just an addon. It can be seen in many of the screen-shots I post here on Wowgoldmillionaire.com. The developer, Munglunch, is friendly and approachable. Mung is actively involved with the users of his work and well worthy of a donation! *hint hint*
http://supervillainui.com/ (under development at time on inclusion)