The full list of changes between the 6.13 and 6.14 releases can be found by reading the 6.14 release notes. A complete list of all bug fixes in the stable DRUPAL-6 branch can be found at http://drupal.org/project/cvs/3060/?branch=DRUPAL-6.
The full list of changes between the 5.19 and 5.20 releases can be found by reading the 5.20 release notes. A complete list of all bug fixes in the stable DRUPAL-5 branch can be found at http://drupal.org/project/cvs/3060/?branch=DRUPAL-5.