Matt Cheney with Chapter 3, a Drupal Consultancy Firm, has create a great list of helpful development tools for drupal. His full list and descriptions can be found on the Chapter 3 site , but here are some teasers:

1. Subversion for Version control
2. Drush for command line control of things like downloading & enabling modules and updating your site
3. Devel Module, Drupal for Firebug, Coder to provide helpful debugging information among other things

For more check out the full article.