Views 2 module concept model
The redesign for version 2 of the essential Drupal module Views was one of my first impactful contributions. Much of the functionality is (still) over my head but somewhere in 2008, after many long conversations in IRC I arrived at a diagram that clarified a lot. I didn’t know of the term concept model then, hence the “sitemap” title.
In simple terms, you use Views to create lists of things. It lets you define what kind of thing to list, where to present it, how to format the list and how to format each item in the list.
What this concept model achieved was mapping the system objects to these simple what, where, how questions. It helped many people understand much better what this new version would be able to do.