Packt has published a new article of mine (extracted from the Learning Drupal 6 Module Development book.) The article, entitled Theming Modules in Drupal 6, discusses how module developers can build modules that take advantage of the robust theme system in Drupal.
From the article's introduction:
The Philosophy Quotes module that we will create in this article will use Drupal's theme system and a simple database query to theme the content of a custom content type.
The goal of the article is to create a module that uses the theme engine to render content. That way, the module will always produce attractively marked-up content. But when a theme designer needs to change the module's look and feel, this can be done using a template... and the module code itself doesn't need to be touched.