The Devel Module is actually a suite of modules designed to assist both module and theme developers. Enable the Devel Module Block to control Devel Module settings.
Theme Developer
This module places markers in the DOM, and is designed to work with FireFox FireBug. Once activated, the module allows theme developers to click on page elements to determine which templates and theme functions effected the element.
Generate Content
Create test content including nodes, comments, terms, users etc.
Node Access Summary
View the node access log for each node on a page.
Performance Logging
View and log performance stats.
Drupal Module Page: http://drupal.org/project/devel