The popular, open-source blogging software WordPress is a powerful tool, and is mostly easy to use. But there are some parts that are not very intuitive, such as customizing output from the database or the site’s appearance. Following is a list of resources I have found very useful in learning how to customize WP’s behavior and appearance. I will continue to add to this list as I find yet more noteworthy resources.

General

The Codex
The Codex is the definitive resource for all official documentation for the WP publishing tool. It is maintained by the WP community, and is constantly being updated and added to. The documentation ranges from WP’s database structure, to functions and methods available, to template tags, to customizing your site’s appearance. It’s all here!
WordPress Support Forums
Possibly the second-most definitive resource after the Codex, the support forums are a great location to ask questions and help others in setting up, customizing, and running WordPress.

Themes, Templates

Anatomy of a WordPress Theme
A brief overview and description of the different files involved in creating a complete theme for WP.
Comprehensive List of WordPress 1.5 Themes
A huge listing of popular themes available for download to use on your site. Browse through and see which ones you like.
Dissection of a WordPress theme
An excellent guide on how to create your own theme for WP. Explains how different files are used to create an overall site. A multi-part series (1, 2, 3, 4).
Deconstructing WordPress’s Comment Template
A discussion of how to customize the layout and appearance of comments left on your blog.
Secrets of WP Theming
An article discussing some lesser-known features of WP with regards to themes, templates, and plugins. A two-part series (1, 2).
WP 1.5 Theme Cheat Sheet
A brief tutorial on how to create your own, bare-bones theme from scratch, and also tells where to go from there for adding more features.

Plugins

WordPress Plugins Database
WordPress comes with a lot of basic functionality. There may be a need to extend the base functionality, or add new functionality. WP 1.5 has been set up in such a way that you can easily add plugins, little modular bits of code to add that little bit extra that you may need. The official plugins listing has quite a few plugins to choose from, to fill many types of functionality.

If you know of any other site that would be a good addition to this list, simply leave a comment here and I’ll make the addition.