Great online tools:

• json.parser.online.fr (Online JSON Parser)

  Excellent for detecting invalid json, and shows both json-parse and eval methods.

• jsonlint.com (JSONLint – The JSON Validator)

  Open-source, has excellent validation of detecting invalid json, and beautifies JSON.

• web.archive.org/…/chris.photobooks.com (JSON Visualization)

  Shows json as html tables and is good for detecting invalid json

• jsonviewer.stack.hu (Online JSON Viewer)

  Nice if you want to traverse json as a tree with properties (but bad for invalid json)

Downloadable tool built on .NET:

• JSON Viewer

  Has a stand-alone viewer similar to the online viewer of the same name, but also has plugins for Fiddler 2 and Visual Studio 2005