X-Cart Mods: Back in Stock Notification X-cart Mod:

"To notify a customer when an out of stock item is back in stock, enter the customer's email address and the product(s) for which they want to receive notification. Back in stock notifications can be added by the customer or administrators. They are automatically deleted when an email is issued to the customer. Customers are automatically notified when the stock is set above 0 quantity in the X-cart admin or via a CSV upload."