Theme developer module (part of the devel module in drupal 6)

For help in theming install the Theme developer module.

For Drupal 6, this module was part of the Devel project. For Drupal 7, it is its own project, but relies on the Devel module as a dependency.

Click on any element to see information about the Drupal theme function or template that made it. Page 400 of Using Drupal, by O'REILLY