A function to use json.tool to beautify JSON.

gistfile1.el

;; beautify json
(defun beautify-json ()
  (interactive)
  (let ((b (if mark-active (min (point) (mark)) (point-min)))
        (e (if mark-active (max (point) (mark)) (point-max))))
    (shell-command-on-region b e
                             "python -mjson.tool" (current-buffer) t)))

To avoid trailing white spaces on lines whose last character is e.g. a comma, maybe the last line of code can be changed as follows:

  "python -mjson.tool | sed -e 's/ $//'" (current-buffer) t)))

This pipes mjson.tool's output into `sed' to remove white spaces at the end of lines.