Update: Version 2 is now available on the Office App Atore.
My first SharePoint 2013 app develop togetter with Anders Dissing has just been released on the SharePoint 2013 app store.
_api JSON Viewer
One of the first things you wish for when working with SharePoint 2013 REST api, is a way to display the raw data, so you know what you have to work with. The _api JSON Viewer app is the tool that does exactly that, with a very user-friendly display and simple functionality. It even allows you to view JSON data across the site collection and every apps. This is a must have app for anyone working with the SharePoint 2013 REST api.
It’s a free app, so go and get it.