Cold Turkey Writer
This app shuts down distracting apps and websites until you have reached either a time limit or word count. Free desktop app for Mac and PC.
Write or Die
Choose from 3 modes, then get writing! If you stop typing, you will get punished in different ways depending on your mode of choice. Available free as a web based tool, although a more detailed paid version is available.

Focus 45
A customizable Google Chrome extension that limits access to distracting sites for 45 minutes at a time.

SelfControl
Site blocker for Mac which allows you to block designated sites for blocks of time.

Coffitivity
Keep focused with ambient coffee shop noise options. There are a variety to choose from.
