JSONTamer is a simple Firefox extension that adds a new sidebar, and a new menu item under Tools. It allows the user to paste in any valid JSON string, as defined by JSON.org. When a string is entered, and the user hits Display, a tree is generated, showing the resulting object.
Current planned features include a version that can be used with the most excellent Komodo IDE, and a set of tools to allow creation of objects from scratch and manipulating existing objects.
A git repository will be available in the near future. For now, JSONTamer is only tested with Firefox 3.0 and above, though may work on older versions.