Plugins are a major part of why WordPress powers millions of blogs and websites around the world.
The ability to extend WordPress to meet just about any need is a powerful motivator for choosing WordPress over other alternatives. Having written several plugins myself, Ive come to learn many (but certainly not all) of the ins-and-outs of WordPress plugin development, and this article is a culmination of the things
I think every WordPress plugin developer should know. Oh, and keep in mind everything you see here is compatible with WordPress 3.0+.