JSON viewer for browsing APIs

I'm not completely sure what you're asking.

• If you're looking for a JSON viewer to examine complex JSON strings, take a look at JSON Visualizer.

• If you're looking for a way to browse an API, consult that API's documentation.

Johns tool is nice. Anoter nice online tool is http://jsonviewer.stack.hu/

This one looks quite good, it's a firefox plugin called JSONView
https://addons.mozilla.org/en-US/firefox/addon/10869

Also ported to a chrome extension
https://chrome.google.com/webstore/detail/chklaanhfefbnpoihckbnefhakgolnmc

Addition
Postman is an awesome stand-alone program for working with APIs
https://www.getpostman.com/