Super Favorite Drupal Modules

As part of a "homework assignment" for the Drupal Nebraska meetup group, we were to pick some of our favorite Drupal modules. I'm sure that I'm leaving a few out but here's a short list off the top of my head:

Acquia Drupal
Okay, I'm starting off here with a big cheat as this is not a module. Instead, it is a Drupal distribution that comes with, by default, a lot of the modules I install on every site. This saves me a nice little chunk of module download and install time. This includes: cck, comment_notify, google_analytics, imagecache, pathauto, token, views, and webform.
SEO Checklist
This module from Volacci doesn't do anything but recommend what SEO-related modules to install. It helps you keep track of what you still need to install and configure.
Vertical Tabs
The bottom of your node edit page gets to look a little intimidating - especially as you install more an more modules. This module gathers all those fieldsets and wrangles them into a nice vertical tab interface.
Auto Node Title
Some handy automation settings for node titles. This is especially useful on content types that need a sensible name for a list but you don't want a user to have to come up with that every time.
PNG Fix
Until our dear old friend IE6 finally passes on, this is an acceptable method to enable full support for PNG background transparency.
Rules
Create some cool workflow actions that fire on specific events. You can do a lot with the default ones or easily create a module that extends the Rules API.
Drupal Message Themer
Not an official Drupal contributed module - just a simple one I whipped up to assist theming Drupal's user notification messages. Many themes, especially custom ones, tend to forget about styling these messages. This is partially due to the fact that they only display under certain circumstances. This module forces them to appear for easier theming.