Drupal 6.12 and 5.18, maintenance releases fixing problems reported using the bug tracking system, as well as a critical security vulnerability, are now available for download. Both releases fix some 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.