Drupal 6.13 and 5.19 released
Drupal 5.19 and 6.13 were released in response to the discovery of security vulnerabilities. There are no new features in these releases, but Drupal security team strongly recommend you upgrade your existing Drupal 5 and 6 sites.

Only fixes for security vulnerabilities and other bugs have been committed. New features are only being added to the forthcoming Drupal 7.0 release. Read the complete release announcement here.
Sites are urged to upgrade immediately after reading the official security announcement: SA-CORE-2009-007 Drupal core – Multiple vulnerabilities.
Drupal 6.13
The full list of changes between the 6.12 and 6.13 releases can be found by reading the Drupal 6.13 release notes. A complete list of all bug fixes in the stable DRUPAL-6 branch can be found here.
Drupal 5.19
The full list of changes between the 5.18 and 5.19 releases can be found by reading the 5.19 release notes. A complete list of all bug fixes in the stable DRUPAL-5 branch can be found here.
Tags: development, Drupal, Hosting and Security, open source