This is just a short post so that next time I don't forget the following.
I've just moved to the Eclipse Mars release and one of the first things I did with the new install was to open a JSON file and find that I didn't have the option to format the JSON from being stretched out on a single line to being prett-printed so I could read it.
I had forgotten that previously I had installed the json-tools package from the marketplace.
You go to the link and drag the button onto your workspace and the install kicks off automatically. Simple! Now I have the right click "Format JSON" option back again.