Python comes with a JSON encoder and decoder you can use to prettify messy JSON code.

On the Command Line

If you have an unindented JSON file:

cat ugly.json | python -mjson.tool > pretty.json

(May I recommend the wonderful DTerm to run commands from almost any app you might be in?)

From within VIM

Using filter commands, !, replace the text with its formatted version, filtered through the same Python command:

:%!python -mjson.tool

Since it’s VIM, you can replace as little or as much as needed; see Use filter commands to process text for more information.

With a Service

Create an Automator service and use the Run Shell Script action to process the input text (make sure to select the Output replaces selected text checkbox), once again using Python:

Since Automator automatically saves services to the ~/Library/Services/ directory, it’ll become available to you instantly, and it’ll allow you to go from this:

to this: