WordPress.org

Plugin Resources

Managing Plugins - How to find, install, manage, and troubleshoot plugins
Plugins - WordPress plugin list, and links to other plugin repositories
Plugins/Plugin Compatibility - List of which plugins are compatible with which versions of WordPress

Writing a Plugin - Best starting place for learning about how to develop Plugins
I18n for WordPress Developers - Internationalization, including a section on how to internationalize your plugin
Plugin API - Description of how to use Hooks (Actions and Filters) in your Plugins, and WordPress functions that plugins can override.
Adding Administration Menus - How to add management menus to your plugin
Plugin Submission and Promotion - Once you have written your plugin, here are some hints on distributing it widely
Migrating Plugins and Themes - Contains information on how to upgrade your Plugin so it will work from version to version of WordPress
Determining Plugin and Content Directories - Example code and explanation of how to determine where your plugin files/folders will be in relation to the server and wordpress install.

External Resources:

Plugin API/Filter Reference - Reference list of filter hooks available in WordPress
Plugin API/Action Reference - Reference list of action hooks available in WordPress
Function Reference - Complete PHP function reference for WordPress (version 2.1, with links to previous versions)
WordPress.org PHPDoc - Nearly all the functions in WordPress now have "PHPDoc" headers. This site shows all the PHPDoc.
Option Reference - Large list of options that WordPress installs by default (version 2.3.1, no links to previous versions)
Database Description - Description of the WordPress Database (version 2.1; has links to older version articles)
Query Overview - Description of the WordPress query process used to find posts and display them
WordPress Coding Standards - General information about coding standards for WordPress development
WordPress Deprecated Functions Hook - WordPress 2.4 adds two hooks for when deprecated functions are used, here is how to use them.
Shortcode API - A tutorial and reference for the shortcode API (new in version 2.5)
Dashboard Widgets API - A reference with examples for adding new widgets to the admin dashboard.
Settings API - A reference with examples for adding new settings to existing settings screens.

External Resources:

PHPXref for WordPress - Cross reference of WordPress files, functions, and variables, which seems to point to the latest released version. This redalt.com PHPXref site has the bleeding-edge Trunk version, as well as 1.5 and 2.0, but the Trunk version may not be updating any more. So, there is now a new Trunk Xref site on ftwr.co.uk available.
Skippy's list of actions and filters
WordPress Hooks Database, a database of all WordPress' hooks, showing which version they come from, and linking to the source code spots that use them
Angsuman's list, a comprehensive listing of WordPress action hooks with documentation and source code location information. It contains all documented and undocumented action hooks in WordPress 2.0.
dd32's wordpress.org/extend Plugin API information
PHP Cross Reference of the WordPress and BBPress source, using PHPXref by Joost de Valk - also includes tutorial
WordPress Plugin Development - Beginner's guide

Creating Admin Themes - How to create a Plugin that changes the look of the Admin section
Creating Tables with Plugins - How to store your plugin's data in a new table in the WordPress database
Custom Queries - How to implement custom queries that let a plugin control which posts are displayed and in what order
AJAX in Plugins - How to add AJAX functionality to your plugin, in the administration screens or viewer-facing pages
Creating Options Pages - How to easily create custom options pages and have WordPress do all the hard work for you!
Modifying Options Pages - How to modify already existing options pages by adding options for your use on them
Using the add_meta_box() Function - How to add a custom section to the post editing screen, and save the data the user enters
TinyMCE Custom Buttons - How to add functionality to the TinyMCE Visual/HTML editor for post/page content

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