The Views module (http://drupal.org/project/views) is used extensively throughout Drupal Commerce for almost all the administration screens and shopping cart / checkout. The fact that almost everything in Drupal Commerce is powered by Views makes it very easy to modify the shopping experience for both the customer and store owner without the need for coding.

The Rules module (http://drupal.org/project/rules) is used in many contexts throughout Drupal Commerce including for the calculation of prices and taxes. Rules is a very flexible and extensible system and provides many options for implementing the rules of your business.

The Entity module (http://drupal.org/project/entity) allows much greater access to the entity data that makes up the Commerce system. This will be of great benefit if you start to develop more comprehensive sites using Rules and particularly if you plan to write your own Drupal Commerce modules.

Instead of defining its own Address field standard, Drupal Commerce utilizes the Address Field module (http://drupal.org/project/addressfield), which uses international standard address formats (xNAL) to ensure that all countries' address standards are supported.

The Chaos Tools (Ctools) module (http://drupal.org/project/ctools) is a utility module that allows Drupal Commerce to implement a common pluggable architecture.

The Token module (http://drupal.org/project/token) provides text placeholders that represent (and are expanded into) entity properties or fields depending on the context of where the token is used. For example, when visiting a user's profile page, the token [user], would represent the user's unique ID (uid) and within the context of an actual item of content such as a page,[author] might expand to the author of the post, that is also the uid property of the item of content.

The Pathauto module (http://drupal.org/project/pathauto) automatically generates URL/path aliases for various kinds of content (nodes, taxonomy terms, and users) without requiring the user to manually specify the path alias.