Drush is a powerfull and must have tools for every Drupal developer. In additions to core Drush commands there are few great modules.
Here is a list of my favourites:
A module which auto-generates a skeleton or "scaffolding" for a module, along with hints on how to fill them in. Useful for newbie developers to learn how hooks work, and seasoned developers who are too lazy to look up what arguments a function has to take. ;)
Provides drush commands to manage roles and permissions.
Drush commands allowing languages to be added, switched, enabled and disabled from the commandline.
You can install Drush modules by putting module in ~/.drush directory.