uc_stock_notify replaces the "Add to cart" button with "Notify when back in stock" when a product is not in stock and the user has permission to use notifications. An email is sent to the users when the product stock level is set above zero on the edit product stock page.
Once the module is installed, the email subject and body can be configured at at /admin/store/settings/stock and the "subscribe to stock notices" permission must be given to the relevant users. Anonymous stock notifications are now supported - users who are not logged in are prompted for their email address.
Workflow_ng integration is included so you can add further conditions and actions for when a product comes back into stock.
uc_stock_adjust() does not provide a hook, so stock level changes are detected by adding a new submit handler into the stock edit form. This means the module is probably not compatible with other modules that alter stock levels. The module is designed to handle multiple SKUs for a product, but attribute support is currently not implemented.
Bug reports, feature suggestions and patches are welcome, but please use the Drupal project page from now on.