Write or Die
It's an online program where you set a time and it will give negative consequences if you stop writing in the window it provides for too long. Lower levels will just start playing an annoying song, but kamikaze mode will actually start deleting your writing. Useful if all you need is a good kick in the pants.
Temptation Blocker
A downloadable program that will lock you out of distracting things on your computer such as games or even web browsers. Provides an unlock code, but it's a 30-something-letter-long string of nonsense to discourage you until you hit your time goal. Good if you find yourself not writing because there are a lot of distractions.
Both of those I used during NaNoWriMo last year and they were really helpful in getting things done for me.