WordPress is easily my favorite open source software project.  I love it for it’s functionality, flexibility, and extensibility.  When it comes to writing a plugin, these are my favorite resources.  Don’t write a plugin without them!

1. WordPress.org Codex for Writing a Plugin
1. If you’re just getting started, this is the place to begin
2. WordPress.org Codex Plugin API
1. a great overview of the WordPress plugin API
3. WordPress Action Reference
• When WordPress displays a post, page, or the administrative pages it has to run through a number of functions and actions.  Your plugin will need to be activated at one of these points, and it is very helpful to know the order in which things happen.
4. WordPress Filter Reference
• The WordPress filter reference is a list of WordPress filters.  Each one will be able to deliver a little piece of the website for your to manipulate in your plugins.
5. WordPress PHP Cross-Reference
• The WordPress codex is pretty good – but it is not comprehensive.  If you want to know how some of the more obscure functions, variables, or constants work, you’ll just need to dive into the source code itself.  PHPXRef is, hands down, the best way to do this.  It let’s you search and read the the source code from their website.
6. Top 10 Most Common Coding Mistakes in WordPress Plugins
• This is quite possibly the best blog post about writing WordPress plugins.  Applying these guidelines will make you a better WordPress developer and your plugins faster, more efficient, and more awesome. :)
7. How to Design and Style Your WordPress Plugin Admin Panel
• In a lot of ways, a program is only as good as its user interface.  Build a good friendly and powerful interface and people will use your program.  Build a bad one and no one will use it, no matter how awesome it is.  This one blog post gives numerous little ways to make your WordPress plugin administrative interface look better.