We have a very wide community of Drupal6 users,contributors and still growing.Here are few quick drupal links for intermidiate,begineeres,and also for advanced drupal coders.
http://drupal.org/getting-started
http://drupal.org/handbook/customization/tutorials
- check out beginners cookbook and step-by-step articles
http://drupal.org/handbook/customization/howto
theme guide for drupal
http://drupal.org/theme-guide
http://drupal.org/handbook/customization/videocasts
http://drupal.org/handbook/site-recipes: That links to HowTo's, Tutorials, Videos, Snippets, etc.
Snippets for customizations: http://drupal.org/node/45471
Video and info on CCK: http://drupal.org/node/200657
How to troubleshoot: http://drupal.org/node/72108
Enhancing Drupal's functionality: http://drupal.org/node/33958
Developing a government website in Drupal: http://drupal.org/node/27661
Pains of supporting a new site: http://drupal.org/node/137898
Best practices: http://drupal.org/node/17557
If you have more suggestions then please comment them below.