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 module intercepts all outgoing emails from a Drupal site and reroutes them to a predefined configurable email address...."

Pretty sweet module. Easily build form based pages. It can have system variables for fields if the Variable module is installed. The forms are exportable. Uses the Form Builder module.

This module is a great way to easily create a form and generate the PHP form structure. Used in the Configuration Builder module.

This module makes it so you don't have to rsync the whole remote files directory on a dev machine.

Created by webchick, this module teaches how to code a module for Drupal 8.

The examples module contains a collection of example modules for developers.

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

Really rad developer module. Use this in addition to Devel to become super awesome. One thing I like about it is the ability to clear any cache you want; you can clear the cache from just one view, for example.

Developing with Apache Solr? Then you pretty much gotta have this installed. It will show you the entire Drupal Solr entity in a subtab next to the normal devel subtabs.