Pressflow is a distribution of Drupal with integrated performance, scalability, availability, and testing enhancements.
Pressflow addresses a long-standing problem: High-traffic sites use stable versions of Drupal, and stable versions of Drupal are ineligible for enhancements to solve performance bottlenecks discovered after widespread deployment.
Like Drupal, Pressflow is free, open-source software released under the GNU General Public License, versions two and later, as published by the Free Software Foundation.