The Custom Order Status app allows you to create unlimited custom statuses for your orders. When you create a status, you have the option of notifying the customer of the change as well as any other custom email address you enter (so you can notify your supplier for example).
The app also has an Order Lookup Page that allows your customers to check on the status of their orders, even if they don't have an account in shopify. They simply enter their email address used when they made the order and the order number.
New Updates! (January 2017)