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.

With that in mind, I put together a Drupal Theme Developer's Cheat Sheet that includes some of the most handy reference information for themers.

Local mirror of Drupal Theme Developer's Cheat Sheet