Format numbers in django templates

I'm trying to format numbers. Examples:

1     => 1
12    => 12
123   => 123
1234  => 1,234
12345 => 12,345

It strikes as a fairly common thing to do but I can't figure out which filter I'm supposed to use.

Edit: If you've a generic Python way to do this, I'm happy adding a formatted field in my model.

Django's contributed humanize application does this:

{% load humanize %}
{{ my_num|intcomma }}

Be sure to add 'django.contrib.humanize' to your INSTALLED_APPS list in the settings.py file.

Building on other answers, to extend this to floats, you can do:

{% load humanize %}
{{ floatvalue|floatformat:2|intcomma }}

Documentation: floatformat, intcomma.

Regarding Ned Batchelder's solution, here it is with 2 decimal points and a dollar sign. This goes somewhere like my_app/templatetags/my_filters.py

from django import template
from django.contrib.humanize.templatetags.humanize import intcomma

register = template.Library()

def currency(dollars):
    dollars = round(float(dollars), 2)
    return "$%s%s" % (intcomma(int(dollars)), ("%0.2f" % dollars)[-3:])

register.filter('currency', currency)

Then you can

{% load my_filters %}
{{my_dollars | currency}}