This is the best JSON Formatter I have found. It makes JSON easy to read, and it is open source.
JSON Formatter Features
• JSON & JSONP support • Syntax highlighting • Collapsible trees, with indent guides • Clickable URLs • Toggle between raw and parsed JSON • Works on any valid JSON page – URL doesn't matter • Works on local files too (if you enable this in chrome://extensions) • You can inspect the JSON by typing "json" in the console
Try it out on this URL // Google Spreadsheet as JSON values
Keyboard Shortcut for JSON Formatter
Hold down control (or cmd on Mac) while collapsing a tree if you want to collapse all its siblings too.