Como fazer download de arquivos pela urllib limitando a banda usada. Usa a biblioteca progressbar (http://cheeseshop.python.org/pypi/progressbar).

{{{
#!python

#!/usr/bin/python
#
# limitget.py
# Copyright (c) 2006 by Nilton Volpato <first-name dot last-name @ gmail.com>
#
# Permission to use, copy, modify, and distribute this software and its
# documentation for any purpose and without fee is hereby granted,
# provided that the above copyright notice appear in all copies and that
# both that copyright notice and this permission notice appear in
# supporting documentation.
#
# This file is provided AS IS with no warranties of any kind.  The author
# shall have no liability with respect to the infringement of copyrights,
# trade secrets or any patents by this file or any part thereof.  In no
# event will the author be liable for any lost revenue or profits or
# other special, indirect and consequential damages.

import urllib
import time
from progressbar import ProgressBar, Bar, ETA, Percentage, FileTransferSpeed

def limitget(url, outfname, limit):
    """Downloads an url to a local file using a bandwidth limit.
    url: resource to download
    outfname: filename to which url will be copied
    limit: bandwidth limit in bytes/second
    """
    fp = urllib.urlopen(url)
    op = open(outfname, 'wb')
    n = 0
    widgets = [Percentage(), ' ', Bar(),' ', ETA(), ' ', FileTransferSpeed()]
    size = int(fp.headers['content-length'])
    pbar = ProgressBar(widgets=widgets, maxval=size).start()
    while 1:
        start = time.time()
        s = fp.read(limit)
        if not s:
            break
        op.write(s)
        n += len(s)
        timetosleep = 1.0 - (time.time() - start)
        if timetosleep > 0.0:
            time.sleep(timetosleep)
        pbar.update(n)
    pbar.finish()
    fp.close()
    op.close()

if __name__ == '__main__':
    import sys
    limitget(sys.argv[1], sys.argv[2], int(sys.argv[3]))

}}}

Outro jeito de fazer é como abaixo. Mas cuidado pois esse exemplo abaixo pode não funcionar como esperado em situações particulares.
{{{
#!python

#!/usr/bin/python
# -*- encoding: iso-8859-1 -*-
import urllib2
from time import sleep

import timing # To debug
from sys import stdout

def limit(www,bps):
    '''Load www address in bps B/s'''
    con = urllib2.urlopen(www)
    bytes = bps/10
    total_bytes = 0
    timing.start()
    while True:
        data = con.read(bytes)
        total_bytes += len(data)
        if not data:
            break
        stdout.write('Loaded %d bytes\r'%(total_bytes))
        stdout.flush()
        sleep(0.1)
        
    timing.finish()
    time_ = timing.milli()/1000.0
    print 'Loaded %dBytes in %0.2f seconds (%dBps)'%(total_bytes, time_, total_bytes/time_)
}}}

----

-- NiltonVolpato