JSONovich by Will Elwood, mjgiarlo

• Syntax highlighting of JSON elements. [since 0.1]
• Indentation according to JSON object/array nesting level. [since 0.1]
• Line numbering based on JSON nodes and brackets, long strings always count as a single line. [since 0.8]
• Collapsible objects and arrays (whole line is clickable, not just a tiny +/- symbol). [since 1.5]
• Subtle zebra-stripes for easier reading. [since 1.7]
• Preserves formatting accurately when copying to clipboard, excluding line numbers. [since 0.9, broken in 1.7, fixed in 1.9.3]
• Currently supports the following MIME types by default:
  
  • application/json [since 0.1] (also legacy unofficial types application/x-json [since 1.8.2], text/json [since 1.8.2] and text/x-json [since 0.1])
  • application/jsonrequest [since 1.6]
  • application/sparql-results+json [since 0.9.1]
  • application/schema+json [since 1.8.2]
  • application/jsonml+json [since 1.8.2]
  • application/rdf+json [since 1.4]
  • application/javascript and variations [since 1.9.2]
  • application/manifest+json [since 1.99.4]
  The above list is customisable by user preferences. [since 1.8.2]
• Supports loading JSON from files on disk with the following extensions by default:
  
  • .json [since 1.8]
  • .srj [since 1.8.2]
  The above list is customisable by user preferences. [since 1.8.2]
• Can override the default HTTP Accept header to either remove application/json or add it with a specific quality factor. [hosts since 1.9, URL match patterns since 1.99.4]
• Options can be changed via the Add-ons Manager in Firefox 7+, Mobile 7+ and SeaMonkey 2.4+. [since 1.9]
• Full support for documents served with unusual character sets as well as the official UTF8. [fixed in 1.9]

See it in action by trying some of the endpoints at JSON Test.

• What's with the weird name?

  JSONovich is named after Pavement member Bob Nastanovich primarily because he seems like a swell guy.

• And the logo?

  That's called an Ambihelical Hexnut, an optical illusion similar to the impossible torus that is the JSON logo.

To report a problem, please create an issue.

Feel free to add your language and send a pull request.

A history of changes and the latest source code are available from our GitHub project.