The Aegir Hosting System is the only fully free and open source hosting system for Drupal. It allows you to manage anywhere from a few sites for a single organization, to thousands of sites across as many concurrent instances of Drupal, on as many servers, and for as many clients, as you need. Since it's all built on Drupal and Drush, it can be customized and extended using all the tools we're already familiar with.
There are many extensions to Aegir available on Drupal.org and Github, however, the process of building these extensions is extremely daunting to someone that doesn't have an intimate familiarity with the inner workings of the system.
In this session, we'll build a complete extension to the Aegir hosting system that will expose a simple mechanism for choosing the environment of a provisioned site (dev, stage, or live), communicate that information to the Aegir task runner, and inject some configuration into the provisioned site's settings.php.