Articles and examples of using entities.

• api.drupal.org/hook_entity_info
• Coming to Examples project: http://drupal.org/node/893842
• Three-part blog series by Ronald Ashri on Entities:
  1. Moving Beyond Nodes (introduction to entities and why they came to Drupal)
  2. What Entities Are, and Where and When to use them
  3. Programming Hello Drupal Entity
• EntityFieldQuery example from Ryan Szrama (see comments also)

Drupal 7 Module Development also has an extensive entity example in the Working with Content chapter.