How do I write JSON data to a file?

You forgot the actual JSON part - data is a dictionary and not yet JSON-encoded. Write it like this for maximum compatibility (Python 2 and 3):

import json
with open('data.json', 'w') as f:
    json.dump(data, f)

On a modern system (i.e. Python 3 and UTF-8 support), you can write a nicer file with

import json
with open('data.json', 'w', encoding='utf-8') as f:
    json.dump(data, f, ensure_ascii=False, indent=4)

To get utf8-encoded file as opposed to ascii-encoded in the accepted answer for Python 2 use:

import io, json
with io.open('data.txt', 'w', encoding='utf-8') as f:
  f.write(json.dumps(data, ensure_ascii=False))

The code is simpler in Python 3:

import json
with open('data.txt', 'w') as f:
  json.dump(data, f, ensure_ascii=False)

On Windows, the encoding='utf-8' argument to open is still necessary.

To avoid storing an encoded copy of the data in memory (result of dumps) and to output utf8-encoded bytestrings in both Python 2 and 3, use:

import json, codecs
with open('data.txt', 'wb') as f:
    json.dump(data, codecs.getwriter('utf-8')(f), ensure_ascii=False)

The codecs.getwriter call is redundant in Python 3 but required for Python 2

Readability and size:

The use of ensure_ascii=False gives better readability and smaller size:

>>> json.dumps({'price': '€10'})
'{"price": "\\u20ac10"}'
>>> json.dumps({'price': '€10'}, ensure_ascii=False)
'{"price": "€10"}'

>>> len(json.dumps({'абвгд': 1}))
37
>>> len(json.dumps({'абвгд': 1}, ensure_ascii=False).encode('utf8'))
17

Further improve readability by adding flags indent=4, sort_keys=True (as suggested by dinos66) to arguments of dump or dumps. This way you'll get a nicely indented sorted structure in the json file at the cost of a slightly larger file size.

I would answer with slight modification with aforementioned answers and that is to write a prettified JSON file which human eyes can read better. For this, pass sort_keys as True and indent with 4 space characters and you are good to go. Also take care of ensuring that the ascii codes will not be written in your JSON file:

with open('data.txt', 'w') as outfile:
     json.dump(jsonData, outfile, sort_keys = True, indent = 4,
               ensure_ascii = False)

Read and write JSON files with Python 2+3; works with unicode

# -*- coding: utf-8 -*-
import json

# Make it work for Python 2+3 and with Unicode
import io
try:
    to_unicode = unicode
except NameError:
    to_unicode = str

# Define data
data = {'a list': [1, 42, 3.141, 1337, 'help', u'€'],
        'a string': 'bla',
        'another dict': {'foo': 'bar',
                         'key': 'value',
                         'the answer': 42}}

# Write JSON file
with io.open('data.json', 'w', encoding='utf8') as outfile:
    str_ = json.dumps(data,
                      indent=4, sort_keys=True,
                      separators=(',', ': '), ensure_ascii=False)
    outfile.write(to_unicode(str_))

# Read JSON file
with open('data.json') as data_file:
    data_loaded = json.load(data_file)

print(data == data_loaded)

Explanation of the parameters of json.dump:

indent: Use 4 spaces to indent each entry, e.g. when a new dict is started (otherwise all will be in one line),
sort_keys: sort the keys of dictionaries. This is useful if you want to compare json files with a diff tool / put them under version control.
separators: To prevent Python from adding trailing whitespaces

With a package

Have a look at my utility package mpu for a super simple and easy to remember one:

import mpu.io
data = mpu.io.read('example.json')
mpu.io.write('example.json', data)

Created JSON file

{
    "a list":[
        1,
        42,
        3.141,
        1337,
        "help",
        "€"
    ],
    "a string":"bla",
    "another dict":{
        "foo":"bar",
        "key":"value",
        "the answer":42
    }
}

Common file endings

.json

Alternatives

CSV: Super simple format (read & write)
JSON: Nice for writing human-readable data; VERY commonly used (read & write)
YAML: YAML is a superset of JSON, but easier to read (read & write, comparison of JSON and YAML)
pickle: A Python serialization format (read & write)
MessagePack (Python package): More compact representation (read & write)
HDF5 (Python package): Nice for matrices (read & write)
XML: exists too *sigh* (read & write)

For your application, the following might be important:

Support by other programming languages
Reading / writing performance
Compactness (file size)

See also: Comparison of data serialization formats

In case you are rather looking for a way to make configuration files, you might want to read my short article Configuration files in Python