Made from about 500 000 hits from an undisclosed servers. Sample usage: 
import urllib
from ua import ua
 
class myOwnUrlOpen(urllib.FancyURLopener):
 def __init__(self, *args):
  self.version = ua.gett()
  urllib.FancyURLopener.__init__(self)

o = myOwnUrlOpen()
f = o.open("http://www.coolwebsite.com/")
doc = f.read()
The sources: 
import random

class ua:
 def __init__(self):
  self.ua = ['Mozilla/5.0 (Macintosh; U; Intel Mac OS X; de-de) AppleWebKit/419 (KHTML, like Gecko) Safari/419.3', 'Mozilla/5.0 (Macintosh; U; PPC Mac OS X; ja-jp) AppleWebKit/419 (KHTML, like Gecko)', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; FunWebProducts; .NET CLR 1.1.4322)', 'Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.4) Gecko/20060601 Firefox/2.0.0.4 (Ubuntu-edgy)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.0.3705)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; FDM)', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1) Gecko/20061010 Firefox/2.0', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; hu; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'kh_lt/LT4.0.2737', 'Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en-us) AppleWebKit/419 (KHTML, like Gecko) Safari/419.3', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; (R1 1.5); .NET CLR 1.1.4322)', 'Mozilla/5.0 (Macintosh; U; Intel Mac OS X; fr; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; pt-BR; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/5.0 (Windows; U; Windows NT 6.0; en-GB; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/5.0 (Macintosh; U; PPC Mac OS X; it-it) AppleWebKit/419 (KHTML, like Gecko) Safari/419.3', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.0.3705; .NET CLR 1.1.4322; Media Center PC 4.0; .NET CLR 2.0.50727)', 'Mozilla/5.0 (Macintosh; U; Intel Mac OS X; de; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; pl; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 2.0.50727; .NET CLR 1.1.4322)', 'Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en-US) AppleWebKit/420+ (KHTML, like Gecko, Safari/420) OmniWeb/v607.17', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; InfoPath.2)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.2; MathPlayer 2.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; FunWebProducts; .NET CLR 1.1.4322)', 'Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en) AppleWebKit/419 (KHTML, like Gecko)', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; fr; rv:1.8.1.3) Gecko/20070309 Firefox/2.0.0.3', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.2) Gecko/20070219 Firefox/2.0.0.2', 'Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.8.0.12) Gecko/20070508 Firefox/1.5.0.12', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; InfoPath.1)', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 2.0.50727; .NET CLR 1.1.4322)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; .NET CLR 3.0.04506; .NET CLR 1.1.4322)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; InfoPath.1; .NET CLR 2.0.50727)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; .NET CLR 3.0.04506; InfoPath.1)', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.8.0.12) Gecko/20070508 Firefox/1.5.0.12', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows 98)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; InfoPath.1)', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 2.0.50727; .NET CLR 1.1.4322; InfoPath.1)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; InfoPath.2)', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; InfoPath.1)', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727)', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; InfoPath.1)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; .NET CLR 3.0.04506; InfoPath.2)', 'Opera/9.21 (Windows NT 5.1; U; en)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.0.3705; .NET CLR 2.0.50727; .NET CLR 1.1.4322; Media Center PC 4.0; InfoPath.1)', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; nl; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30)', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322; InfoPath.1)', 'Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.8.1.3) Gecko/20070309 Firefox/2.0.0.3', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.1.4322)', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; fr; rv:1.8.0.12) Gecko/20070508 Firefox/1.5.0.12', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.0.3705; .NET CLR 1.1.4322; Media Center PC 4.0; .NET CLR 2.0.50727)', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0; .NET CLR 1.1.4322)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; InfoPath.1)', 'Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.8.1.3) Gecko/20070309 Firefox/2.0.0.3', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.0.3705; .NET CLR 1.1.4322; Media Center PC 4.0)', 'Mozilla/5.0 (compatible; Yahoo! Slurp; http://help.yahoo.com/help/us/ysearch/slurp)', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; it; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.0.11) Gecko/20070312 Firefox/1.5.0.11', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322; InfoPath.1; .NET CLR 2.0.50727)', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)', 'Mozilla/5.0 (Macintosh; U; Intel Mac OS X; es) AppleWebKit/419 (KHTML, like Gecko)', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.0.3705; .NET CLR 1.1.4322)', 'Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 2.0.50727)', 'Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en-US; rv:1.8.1.3) Gecko/20070309 Firefox/2.0.0.3', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 2.0.50727)', 'Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.4) Gecko/20061201 Firefox/2.0.0.4 (Ubuntu-feisty)', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.0.3705; .NET CLR 1.1.4322; Media Center PC 4.0)', 'Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.3) Gecko/20061201 Firefox/2.0.0.3 (Ubuntu-feisty)', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; ja; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; es-ES; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; SLCC1; .NET CLR 2.0.50727; .NET CLR 3.0.04506)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; .NET CLR 3.0.04506; InfoPath.2; .NET CLR 1.1.4322)', 'Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en-US; rv:1.8.0.12) Gecko/20070508 Firefox/1.5.0.12', 'Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en) AppleWebKit/419 (KHTML, like Gecko)', 'Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en) AppleWebKit/419 (KHTML, like Gecko) Safari/419.3', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; .NET CLR 3.0.04506)', 'Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en) AppleWebKit/419 (KHTML, like Gecko) Safari/419.3', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1)', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.3) Gecko/20070309 Firefox/2.0.0.3', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.0.12) Gecko/20070508 Firefox/1.5.0.12', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)', 'Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en-US; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', '   9003 ', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322)', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322)', 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30)', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; fr; rv:1.8.1.4) Gecko/20070515 Firefox/2.0.0.4', 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)']
 def gett(self):
  return self.ua[random.randrange(len(self.ua))]