Security Kit

"Module provides Drupal installation with various security hardening options. This lets your mitigate the risks of exploitation of different web application vulnerabilities."

Panels

A feature packed layout tool that grows on you. Used by many many websites.

Rules

Rules is an amazing module with so many features. Among other things it has the ability to send out emails and other kind of notifications based upon all kinds of conditions - just about any kind of notification system can be built with Rules as a foundation. But anyway, just go check it out. Check out the Johan Falk at NodeOne Rules screencasts if you want some good tutorial videos.

Panels Everywhere

Let panels take over the whole page. From d.o:
"Panels Everywhere is an advanced method to completely do away with Drupal's restrictive blocks system and instead use the much more freeing Panels Layout system to control how your pages look. Panels Everywhere modifies the page as it is being rendered to 'wrap' the content in a display and can even take over your page theme to do away with the need for a page.tpl.php."