Module Builder, Drushify, Coder, and API, are four great Drupal 7 contributed modules which make a module developer's job much easier.
Module Builder - https://drupal.org/project/module_builder A module which auto-generates a skeleton or "scaffolding" for a module, along with hints on how to fill them in. This can be incredibly useful for less experienced Drupal developers be providing some examples on how hooks work and where files are organized and structured for Drupal 7 modules.
Even seasoned developers can get something out of it, like those who know how to construct a module but don't feel like looking up what arguments a specific function requires. Module Builder integrates with Drush, making the scaffolding process for module development a breeze. This one is a favorite of mine, since a good portion of my job is making custom modules.
Drushify - https://drupal.org/project/drushify drushify is a simple code generator that creates a skeleton Drush commandfile template for the module you specify. This is really helpful if you want to provide Drush integration and commands with your custom module. While seemingly a small deal, this kind of integration is very impactful on the Drupal community. Anything that simplifies the process of broadening Drush interactivity within the Drupal contributed module ecosystem is a big win for everyone.
Coder - https://drupal.org/project/coder The Coder project includes two developer modules that assist with code review and code manipulation. Each of the modules supports a plug-in extensible hook system so contributed modules can define additional review standards and upgrade routines. The code evaluation tool is useful for making sure you keep your module code in line with Drupal coding standards.
If you plan on submitting a new contributed module to the Drupal community, using Coder should probably be considered a required step in the process. But even if you are just creating some custom modules for an internal project, the Coder module can help you keep it within Drupal standards and avoid security pitfalls in your code.
API - https://drupal.org/project/api The API module was designed to produce the Drupal developer documentation available at http://api.drupal.org. The API module implements a subset of the Doxygen documentation generator specification, with some Drupal-specific additions. With that, you can set up your own API documentation site. If you are the maintainer of a complicated Drupal contributed module, this can be a great way to provide easy access to searching the hooks and functions that are provided by your code.