I often forget that not everyone knows these development best practice buzz words.  As more and more developers join our ranks it will be critical to make sure everyone is developing at full potential.  Here’s a list of some MUST have modules when developing any site:

• Hacked - Get feedback about how modules on a site are different from their drupal.org originating source. Here’s a great article about Drush integration.
• Diff - Utility module that provides diff capabilities to Hacked, node revisions, features and other modules.
• Devel - Debug tool for viewing objects in Drupal site while actively developing it.
• Coder - Audit your code against Drupal best practices and use the tool to teach yourself to be a better programmer.
• Coder Upgrade / Drush - Patch to provide Drush integration with the Coder automated module upgrade routine.
• Coder Tough Love - For crazy people that need every white space to be correct, highly recommend a couple times.
• Boomtower Livesite - Convert traditional SQL to Drupal 7+ based PDO layer statements.
• Drush - Command line Drupal.  Has some additional setup server side but will save you hours.
• Simpletest - Simple Test framework for developing test routines for automated system code verification.
• Features - Bundle up site configuration settings for 
• Backup and Migrate - Create jobs to back up your database tables and files at different intervals

Please feel free to add other must haves in the comments below!