December 16th, 2009
Drupal 6.15 and 5.21, maintenance releases which fix issues reported through the bug tracking system, as well as non critical security vulnerabilities, are now available for download. Both releases fix other smaller issues as well.
Upgrading your existing Drupal 5 and 6 sites is strongly recommended. There are no new features in these releases. For more information about the Drupal 6.x release series, consult the Drupal 6.0 release announcement, more information on the 5.x releases can be found in Drupal 5.0 release announcement.