When working with JSON strings, it is usually hard too understand their hierarchy at the first look.

JSONmate is a web app that can beautify (and uglify) JSON strings where these strings can be directly pasted or loaded from a  remote URL.

JSONmate

Also, it has a very nice visual editor for customizing the data and a visualizer that provides a different view for it.

JSONmate has support for JSONP too and it is an open source project.

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