Distinct highlighting of keywords vs values, JSON-specific (non-JS) warnings, quote concealing.
Customization of Jeroen Ruigrok van der Werven's vim-json highlighting script with Rogerz Zhang's indent script.
Pathogen-friendly. Vundle-friendly too. (Or install it manually.)

• Added distinct highlighting for keywords vs values! (This is what made me start this plugin.)
• Added concealing of double quotes, for a minimalist CoffeeScript-inspired look (CSON!).
  • 
    Strings are colored differently than numbers & booleans. The disambiguating purpose of double quotes is thus achieved with colors for a cleaner look.
  • This requires Vim 7.3+. To disable it add let g:vim_json_syntax_conceal = 0 to your .vimrc.
• Added folding of {...} and [...] blocks. To enable it :setlocal foldmethod=syntax (do it permanently on the ftplugin/json.vim file).
• JSON-specific warnings (red highlights):
  • Warn about no-quotes in keywords and single-quotes in keywords and values.
  • Warn about decimals smaller than 1 that don't start with a 0 (.1 gives a warning, it should be 0.1).
  • Warn about comments // and trailing semicolons ;.
  • Warn about missing commas between elements of an object and elsewhere.
  • Warn about trailing commas after the last element in arrays or objects.
  • (All warnings can be turned off with a let g:vim_json_warnings=0 in your vimrc.)
• Recognize .jsonp file type. In JSONP, the wrapping function call at the beginning and the closing semicolon are recognized.

No syntax coloring: wall of text.

Javascript (or for that matter, the standard json.vim) syntax coloring: Barely an improvement from no syntax coloring.

Better JSON syntax coloring: a more meaningful, distinct and elegant presentation.

Here's 2 compelling reasons:

1. All JSON is Javascript but NOT all Javascript is JSON. So {property:1} is invalid because property does not have double quotes around it. {'property':1} is also invalid, because it's single quoted while the only thing that can placate the JSON specification is double quoting. JSON is even fussy enough that {"property":.1} is invalid too, because you should have of course written {"property":0.1}. Also, don't even think about having comments or semicolons, you guessed it: they're invalid. The point being that your syntax highlighter should warn you about these errors, in realtime, which is something that the Javascript highlighter doesn't (because in Javacript they're not errors!).

2. Distinct highlighting for keywords. JSON is an extremely lightweight data format but at its core lies an inescapable conceptual distinction: there are keywords and there are values. There's nothing much to the format besides that, so we might as well display keywords and values differently. This is something that gets lost with Javascript-inspired syntax highlighters, which see keywords as just another string since JSON requires them double quoted. So JSON files remain an impenetrable, indistinct wall of text.

This is the expected behavior of the plugin:

Most trouble, little as it is, has to do with Vim's newfangled concealing, which most people aren't yet familiar with, as it was introduced as recently as Vim 7.3+. If you just don't care for concealing you can easily disable it adding let g:vim_json_syntax_conceal = 0 to your .vimrc.

Concealing is nice for viewing but when you want to edit it should get out of your way seamlessly so you can see the actual code. Thus the default behavior should be text shown normally on the line your cursor is at, on all modes (normal, visual, insert). If this isn't the case and the concealing doesn't go away (not even in insert mode), you most likely have an interfering plugin. You need to look at your concealcursor setting (which can be set through this plugin with g:vim_json_syntax_concealcursor). The specially overeager indentLine, plugin would require yet an additional let g:indentLine_concealcursor="" in your .vimrc as detailed here.

It's a good idea to test drive with the files json-test.json and jsonp-test.jsonp first thing.

• JSON Formatter: Chrome extension for printing JSON and JSONP nicely when you visit it 'directly' in a browser tab.
• Convert JSON to YAML: side by side, real-time conversion of the "fat and rigid" JSON to the "skinny and flexible" YAML.