Work by Pressflow

Pressflow

Pressflow is a distribution of Drupal with integrated performance, scalability, availability, and testing enhancements.
Each version of Pressflow is API-compatible with the same major Drupal version. For example, Pressflow 6 is compatible with all Drupal 6 modules. Pressflow 6 also integrates the SimpleTest system from Drupal 7 and the CDN support patch. See the wiki (linked below) for more documentation.

http://pressflow.org/