Modules to keep site builders and developers happy

Features (
Provides feature management for Drupal. Features provides a UI and API for taking different site building components from modules with exportables (CCK, Views, etc.) and bundling them together in a single feature module.
Strongarm (
Requires Chaos tool suite.
Gives site builders a way to automatically override the default settings that Drupal core and contributed modules ship with.
Chaos tool suite (
A library of helpful tools by Merlin of Chaos, for use by other modules.
Drupal for Firebug (
Requires Firefox plugins Firebug and Drupal for Firebug.
Extends the Firefox Firebug module to provide Drupal specific debugging and status messages.
Devel (
Requires Menu.
Various blocks, pages, and functions for developers.
Drush (
Not a module, but makes site developers & upgraders very happy. (Stanford users: Drush is installed system-wide on Provides "a command line shell and scripting interface for Drupal, a veritable Swiss Army knife designed to make life easier for those of us who spend some of our working hours hacking away at the command prompt."
SimpleTest (
Provides a framework for unit and functional testing.(This functionality is part of core in Drupal 7.)
Security Review (
Provides site security and configuration review assistance. (Stanford users: Note that Security Review does not take into account AFS-specific file protection, and so may flag as insecure files that are actually properly protected under AFS.)
Hacked! (
"Scans the currently installed Drupal, contributed modules and themes … and determines if they have been changed." In other words, helps you figure out where exactly the previous site developer** changed code they shouldn't have. (**You & I, of course, have never hacked core, cough, cough…)