Drupal

General Development
Getting started with Drupal from IBM This REALLY explains how Drupal Works, use the Node-building sequence when implementing your hooks.
Drupal handbooks - Module developer's guide - PHP Snippets - 6x Core Hooks Cheat Sheet 
Search Options - Core templates and suggestions (naming)  AHAH Tutorial - Schema API 
Drupal 6 JavaScript API  

Contributed Modules
Contributed modules for developers  
Views documentation - Views 2 APIs - Documentation on Views Integration and here 
Contributed Modules - Contributed Modules API - CCK Group - CiviCRM 

Contribute to documentation Help other users 

If you install the advanced_help module the index is in admin/advanced_help/  The admin menu has it's own top level entry

Ubercart
Ubercart APIs - Documentation - Catalog discussion 

Drupal Companies Websites
Raincity Studios - Poplar - Aquia - Lullabot - 11 Heavens - Group 42

Reviews
Similar Module 

IRC
Channel List