Commerce Bulk Product Creation was the first contributed module to the commerce project and provides a quick way to create multiple combinations of products with a single form submission. It also optionally manages the creation of the display associated to the product variations in different ways such as generating a display for each product created in bulk.
Commerce Module Tuesday
Commerce Stock is a great demonstration of how Drupal Commerce can be extended with great Drupal 7 features. It adds an integer field for "stock" to a product, and then manages that using rules.
To use Commerce Stock, just enable the module and then visit admin/commerce/config/stock to configure which product types it should be configured for. At this point, a stock field will be added to your products and you can set the initial stock value.
Commerce File is an outstanding and mature module that allows you to sell downloads. It uses Drupal 7's private file system and provides a field that can be added to a product for the downloadable item. If you're interested in Commerce File, you may also be interested in the Commerce Downloads distribution, which comes with Commerce File completely preconfigured for use.
Here's how you set up to sell downloads with Drupal Commerce:
Discounts in Commerce have generally required configuration using rules, and have often required hand-applied configuration and wired-in discount amounts, which is probably out of reach for many site managers. In the Commerce Discount Campaign Feature I've put together a few rules with a content type so that discount amounts and dates can be done just by editing a node. This screencast tells how.