- http://api.drupal.org/api/drupal (The place to check out the code of the core, I am lazy, I would rather open the link, then check the code through my machine)
- http://drupalcontrib.org/api/drupal (Has the code for certain contributed module, again, I use it out of laziness)
- http://drupal.org/project/usage (If you want to use a contributed module, and run into similar module, check the page for usage, go with the popular trend )
- http://drupal.org/forum (drupal forums)
- http://drupalsn.com/ (The social networking site for Drupal, use it to learn, showcase, etc)
- http://drupal.org/documentation/develop (Drupal Development books)
- http://themegarden.org (Showcase for Drupal themes)
E-book- Pro- drupal development is one of the best to start learning drupal.
This is my list, if you know more, comment, and I will post it here after checking the site.