Devel

Essential module for developers. Should be disabled in production environments.

Security Kit

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

Strongarm

Makes site variables exportable. Uses Ctools. Usually used in conjunction with the Features module.

Diff

This module integrates with so many other modules but out of the box can show you detailed node revision diffs.