If you get the following error, when downloading the corpus for NLTK,
[nltk_data] Error loading all: HTTP Error 401: Authorization Required
you need to update the current data server setting in "/usr/lib/python2.7/dist-packages/nltk/downloader.py" file on line 370 from
DEFAULT_URL = 'http://nltk.googlecode.com/svn/trunk/nltk_data/index.xml'
to
DEFAULT_URL = "http://nltk.github.com/nltk_data/"