Most common words in a text file of about 1.1 million words
This is actually quite good. Good use of the collections module. 


One improvement I can think of is switching to the with open(...) as: structure, that way the file will automatically be closed when finished processing.
Secondly it is standard in python to use a if __name__ == '__main__': block.
Lastly sys.argv will work ok for programs with not many arguments, but I'd recommend looking at the argparse module.