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

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.

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."

Dave Reid's alternative to Display Suite and other display modules.

Written by John Albin, this module give you more control over the display of menus. It allows you to place the same menu (or different menu) in different regions and customize exactly which levels are shown in each block.

Maintained by Dave Reid, Earl Miles, Damien McKenna and Derek Wright

Semantic Panels let's you control the markup of panels.

This module give each user the ability to choose which language the admin interface is in.

This module uses the FitVids.js library for fluid width video embeds (e.g. flash video in <iframe>s). You don't need it for pure HTML5 videos.
It supports YouTube, Vimeo, Blip.tv and Kickstarter by default, and can use it with other video providers like Dailymotion or Viddler.

This module automatically saves form states using jQuery Sisyphus