In-stock Alerts is an extension for Chrome that will let you know when an item you've been eyeing finally comes back in stock at your favorite retailer. It uses the free shipping alerts for that product.

In-stock Alerts is currently supporting Amazon.com, Sam's Club, and Target. 

When you hover your mouse over the "Notify Me" button on a product that is not in stock, you can click on it to subscribe you to receive the online order notification. You don't need to complete an order to subscribe. If you receive an email from Target or Amazon, you can buy it right then.

After you receive the notification, you will be able to buy the product. 

You will be able to get the notification on the online order form for the product, but you will need to enter your email address to subscribe to the notifications.