Read JSON from standard input and writes formatted JSON to standard output. Requires Node.js.

formatjson.js

#!/usr/bin/env node

// Reads JSON from stdin and writes equivalent
// nicely-formatted JSON to stdout.

var stdin = process.stdin,
    stdout = process.stdout,
    inputChunks = [];

stdin.resume();
stdin.setEncoding('utf8');

stdin.on('data', function (chunk) {
    inputChunks.push(chunk);
});

stdin.on('end', function () {
    var inputJSON = inputChunks.join(),
        parsedData = JSON.parse(inputJSON),
        outputJSON = JSON.stringify(parsedData, null, '    ');
    stdout.write(outputJSON);
    stdout.write('\n');
});

Here's a oneliner that does the same thing (at least in mac and linux land, not sure about windows):

node -p 'JSON.stringify(JSON.parse(fs.readFileSync(0)),null,2)'

notably, JSON.parse can work off of a buffer and fs.readFileSync(0) reads the zero file descriptor, which is standard input.
Then, node -p is a way to execute and log the output from a statement. You could also write it with a node -e 'console.log(...)' if you would rather be in control of when or how the logging happens.