WordPress plugin development links

While developing Basic-SEO I've found the following sites very useful for reference purposes:

Wordpress Plugin Repository - http://wp-plugins.org

You can't beat learning by example, here you can browse the source code of all the plugins WordPress.org hosts.

WordPress PHP Documentation - http://phpdoc.wordpress.org/

All the comments from the WordPress source code extracted in HTML format and correctly linked, very useful for looking up functions.

WordPress Hooks Database - http://adambrown.info/p/wp_hooks

Database of all the action & filter hooks that have ever been available in any WordPress release and where it occurs in the source code.

WordPress Codex - http://codex.wordpress.org/

The official documentation for WordPress, has a very useful developer documentation section.