Module development and maintenance in Drupal
There are two useful ways to extend or change Drupal. This is through the module and theme systems. A module is code which implements capabilities while a theme changes how the site looks.
Coder (code review) is a module that does an excellent scan for code quality problems. It incorporates hints from the development guidelines and instructions for migrating modules from one release to another. It gives you a list of specific issues to address. It is like 'lint' and other kinds of static analysis tools.
Devel offers many useful doodads that help developers do their job.