Download : Drupal 6 Api Cheat Sheet

Cheat Sheet : Drupal Theme Developers Cheat Sheet

Cheat Sheet : Drupal 5 Forms API Cheat Sheet

Info : Drupal White papers , Cheatsheets and Free books

If you have started with drupal and want to expand your knowledge, you either need to sit and spend time on Drupal Handbooks or buy some Drupal books.
Here I am giving some good collection of PDFs available on the web, which can be used for beginner or intermidiate developers.

Info : Drupal Theme Developer’s Cheat Sheet

The Drupal Handbooks are an exhaustive resource for learning all things Drupal. Unfortunately, sometimes finding just the information you need can be difficult. The Theme Developer’s Guide alone includes 14 sections, most made up of additional sub-sections.