The Drupal Hacker's Cookbook
This is a guide for hackers -- php developers who are looking for small solutions to big problems. This is not a resource written for programming noobs, though programming noobs are encouraged to use it and learn from it. The format is to the point: a problem, a function, and how this function is used as a solution. We'll only cover drupal 6 and beyond.
Expect this guide to grow day by day.