When I did the NaNoWriMo challenge last year, I found that I didn't benefit much from specialised writing tools. But if you need an incentive to keep pumping out words, Written Kitten might do the trick. For every 100 words you type, you get a fresh picture of a cat.
Beyond that, it's just a text input box, so you'll want to copy the results of your hard work and save it somewhere else. If you're a fool for all things feline, though, it might be a handy tool.