Our good friends at Shopify recently released a developer platform which makes it crazy easy to build custom functionality into an e-commerce store using a standalone Rails app. There are already some great apps available in their app store, many of which are running on Heroku.
Check out the excellent getting started video by James MacAulay. It shows just how slick the Shopify API is – these guys are really taking e-commerce to the next level. (And bonus points for use of config vars to store API keys!)