In case you haven't seen it yet, Shopify has a brilliant developer platform that makes it easy for developers to create Shopify add-ons that can be marketed to Shopify store owners. And now, for even more awesomeness, there's the Shopify App Rails Kit that makes it incredibly easy to build an app to run on the Shopify platform. Check out the announcement on the Rails Kits blog, and see how quickly you can build a Shopify app.
Let me know when you've launched your app, and I'll add you to the Shopify App Rails Kit product page as a reference site. :)