After I published my OS X Automator Service for Reformatting JSON Text, one commenter said that he always uses http://jsonformat.com/. I have used that, but I hate the output it produces. Its output is more readable than JSON with all the whitespace stripped out, but not by much.
It also seemed a little silly for such a web page to require a back-end service to do the work. I figured I could whip up a simple web page that wouldn't require any server round-trips to do its work, and that produced reformatted output that wouldn't make me cry.
Here it is: JSON Reformatter
You can look at the source here: https://gist.github.com/kristopherjohnson/5073681
Update 2014-05-29: New version, implemented using AngularJS: http://undefinedvalue.com/2014/05/28/web-page-reformatting-json-text-using-angularjs