White paper: The Path to Drupal
How “simple” and “extensible” equate to high levels of community involvement for open source
Flexibility and participation
Drupal is an open source web application that is known for its flexibility and for the large number of contributors who are involved in its development. Other details, like that it is written in PHP, requires a database, runs on a web server, and specializes in social publishing are interesting, but the first two points are the most important for this discussion. It is flexible, and lots of people are involved in writing Drupal code.
How flexible is Drupal? For being an actual web application and not a pure framework, it is very flexible. Drupal offers packaged solutions for the typical problems faced by web applications; authentication, sessions, permissions, form building and submission, email handling, and so forth. Yet it does so in a way that allows developers to replace or modify virtually every aspect and detail. The diversity of web sites that have been created using Drupal is the best testament to its flexibility. These sites range in nature from large media websites to activist sites, and software projects. No complete list exists, but you can browse an interesting selection at http://buytaert.net/tag/drupal-sites.