From the command-line:

echo '{"one":1,"two":2}' | python -mjson.tool

which outputs:

{
    "one": 1, 
    "two": 2
}

Programmtically, the Python manual describes pretty-printing JSON:

>>> import json
>>> print json.dumps({'4': 5, '6': 7}, sort_keys=True, indent=4)
{
    "4": 5,
    "6": 7
}