WordTarget is a Mac OS X app designed to help you stick to a writing goal. It doesn't matter where or what you are writing - if your goal is to write a set amount of words a day, WordTarget will help you track yourself against that goal, by sitting in your menubar, and monitoring your word count across applications. You can start and stop it if you don't want something to count (like responding to emails or chatting on IM), and you can set it to count up to your target, or down from it, depending on what gets you more motivated!
This is an app that I built and launched myself.
You can see more about the app on its page here.