I was shocked to realize that I had never blathered on about Dr. Wicked's wonderful 'Write or Die' web application. It is something I use weekly to get me through tough patches of not writing.
It's deviously simple. If you don't write, the application punishes you. Gentle mode is pretty gentle and not very effective (for me at any rate). Normal blasts you with annoying music for not producing. Kamikaze mode actually erases words if you quit writing. This last one is extremely effective.