Apptha -
Magento Out of Stock Extension
The Out-of-Stock Notification is a Magento extension which allows the store owners to show a message for the Out-of-stock products, indicating the user to request for notification e-mail once the product is updated in the stock inventory. When stock is not available, then out of stock notification option feature is displayed in the product's view page.
When the custom stock status of the product becomes available, user can receive a notification email.