If you’re looking for a way to prototype an interface, especially one that would benefit from interacting with live data, having a lightweight and easy to use/learn backend is essential. Sinatra fits the bill nicely. It is a Ruby framework for developing web applications. A List Apart has a great tutorial on prototyping with Sinatra, which will get you up and running pretty quickly.
Recently, I needed to cobble together a quick web service to support a hardware project I am working on. The above mentioned tutorial saved me a lot of time. Once I had the service running locally, it was easy to get it deployed to Heroku where the web service was no longer dependent on my laptop running the webserver.