"Ctools Plugin Stub is a Drush plugin that can help you generate code stub files for ctools plugins."

This module is basically an advanced devel generate (see the devel module). Gives more control over the dummy content that gets generated by devel. (I haven't used it yet but it seems promising.)

This is the module that is used on this site for code highlighting.

Firebug for Drupal themeing. See the screenshot and screencast, http://drupal.org/node/209561 . Note that this module injects markers into the DOM to do its magic. This may cause some themes to behave erratically and less capable browsers may make it worse (especially IE)/. Enable it when needed, and disable it afterwards.