If you are starting with WordPress Plugin or theme development, or want to know where to start then here is a small list of articles that you should read, some of them should be always open in your browser, and some of them in your RSS reader.
Documentation and Help
1) http://codex.wordpress.org/Function_Reference: This article list almost all the core functions that are available for a plugin developer.
2) http://codex.wordpress.org/Plugin_API/Action_Reference: This article is a complete list of the action hooks available for use in plugin development.
3) http://codex.wordpress.org/Plugin_API/Filter_Reference: This article is a complete list of the filter hooks available for use in plugin development.
4) http://codex.wordpress.org/Data_Validation: This article is list all the function are available in WordPress core to help us with data sanitization and validation
5) http://codex.wordpress.org/Site_Architecture_1.5: This article describes the general site architecture of a WordPress theme to help you understand it.
6) http://wordpress.stackexchange.com: This is *the* place to ask questions related to WordPress development, and off course give answers as well.
7) http://iwebask.com/blog/2011/03/16/75rss-feeds-for-web-developers-wordpress-bloggers/: Whole bunch of links that you should follow on your RSS reader, so that you can stay in sync with latest WordPress tips and tricks.
8) http://www.w3avenue.com/2009/09/28/definitive-list-of-free-wordpress-theme-frameworks/: This article list whole bunch of theme frameworks that you can use for your themes.
WordPress Core Community
1) http://wpdevel.wordpress.com/ : This is the place where people in the core hang out, read it so that you are upto date with latest happenings about WordPress
2) http://make.wordpress.org/themes/: This is where people who review themes submitted in the wordpress.org theme repository hang out.
Well these are the links that i have found very useful, and I spend most of my time there. If you have any such useful link please share in the comments below.
P.S. We are hiring WordPress developers, email me your resumes if you are interested to become part of the team.