Freedom is the most useful writing tool I’ve come across.  It blocks internet access for the length of time that the user chooses, and one can only get around it by restarting the computer.  Being particularly prone to distraction, I utilize it whenever I write.  I got 7 pages done this morning because of it.  The first 10 uses are free, then it costs $10 to purchase an activation code.  Absolutely worth it.