Whenever a product is out of stock, having an email form field in place of the "add to cart" button that customers can enter their email address in to receive emails when the product is back in stock.
There are other apps that do this, obviously, but I personally love cartstack with all of my heart, so I would prefer if they did it.
I think one of cartstack's greatest strengths is the reporting dashboard, and I love having all of the analytics in one spot.