The fifth alpha release of what will become Drupal 7 has been made available. You can read about the full list of incorporated changes between version 6 and 7 here: http://drupal.org/drupal-7.0-alpha1
From the announcement:
This release includes many critical bug fixes, especially around the upgrade path, security issues, JavaScript, performance, and other areas.
Please see the release notes for more details.
As always, you can download the release from http://www.drupal.org