I was running through the Drupal project directory today (looking for a module) and I came across a neat module called Examples for Developers. Take a look:

http://drupal.org/project/examples

It just contains a bunch of modules that give examples on how to do certain things within Drupal. These are things like creating blocks, using menus and interacting with triggers. The items in this module aren't meant to be run, but it's a great idea that's definitely worth a look.