This is an example application to demonstrate (and in some ways to guide the development of) the sage_pay gem I'm building to interface with Sage Pay's server interface.
You can find a live instance here: https://sage-pay-rails-example.heroku.com/. It will guide you through the Simulator interface, allowing you to pass all kinds of horrible errors back to the app and torture it to see what happens. Hopefully, it works. :-)