David Strauss from Four Kitchens talks about the special patched version of Drupal that he has been maintaining that runs the Drupal.org website. Drupal.org is the hub of activity for the Drupal community, and probably the most active community that is powered by the Drupal software.
There are a number of performance and scalability patches that were written specifically for Drupal.org, which have since gotten committed to Drupal 7, and that have also been backported to Drupal 6. David is maintaining that set of patches in a performance optimized version of Drupal 6 in what is called Pressflow.
David talks more about the features of Pressflow Drupal in the podcast and how the Drupal that runs Drupal.org actually has more in common with Pressflow Drupal than "regular" Drupal that most people download.