Panels for Drupal
I finally got around using panels (5.x version) extensively and experimenting with it. The module already exists quite some time, but it's only recently that the 5.x version got to the release client status. (Drupal 6 version is in alpha).
What is Panels?
Panels is a massive module and my guess is it will be up there with Views and CCK. However, like CCK and Views it is a complex module and can be hard concept to grasp. The description on the panels project page is pretty vague (as it often is).
The best way I would describe it would be: think of panels like an easy way to accomplish some of the things you would now solve with regions and blocks. It enables the possibility to change page layouts without theming your ass off. If you do a lot of theming - Panels is good news.
You can also read what people think of this module at drupalmodules.com.
Take a look at this video. At about 1:15 in the movie you will see some quick drop-and-drag actions which show you how easy it would be to reorganize content on a crowded frontpage. (by the way the whole video really shows the possibilities of Drupal).
Drupal is a powerful framework, but together with the combination of CCK, Views and Panels you can literally build almost anything (yes, just like with LEGO).
Another example which I found even more impressive is this tutorial: howto override and re-layout the edit content screens (like in your admin theme for a node). These are really things which make it easily possible to put stuff where you want it. Not where Drupal initially planned it. And therefore it offers great possibilities to improve usability.