While the Administration Menu module is one of the most-used Drupal modules (even finding its way into the Acquia distribution of Drupal), there's a new kid on the block that's turning a lot of heads.
The Admin module is way more than just a nice menu that sits at the top of every page of the site (although it has that too). It changes the look of the entire administration area of the site (anything with a path that starts with /admin) in a way that makes site admins more efficient.
This module is more than just an administrative theme, it provides quick access to common tasks while still providing access to all of the administrative functionality of the site.
The UI changes being worked on for Drupal 7 already appear to be taking its cues from this module (or is it vice-versa?) - if using this module is any indication of what's coming, I can't wait.