Yarny is one of the best tools I've found for my writing. It was designed by writers with NaNoWriMo in mind. With Yarny, you are writing in the cloud. Your writing is saved automatically, with auto-versioning. And did I mention it's free? Check out their website to learn more about their features.
I found Yarny when I participated in my first NaNoWriMo in 2011 to write Dogs of War. I was able to write from anywhere that had an Internet connection. I also exported my writing to my laptop so I could work on it offline if necessary. When I had a connection again, I just copied and pasted the new text into Yarny. I didn't really have time to learn all the features of Yarny that first year. I didn't take advantage of the snippets to divide my work into chapters and subchapters. I didn't realize until I was editing several months later that Yarny will export the snippet titles as chapter titles. I used it again for 2012 to write Dogs of War: Infiltration, knowing more about what I was doing. I also upgraded to the paid Premium version of Yarny. It's not really necessary, but it allows you to save multiple titles. With the free version, you can only have one writing project in the cloud at a time. I upgraded so I can keep all of my writing together and edit one while writing another, etc.
Yarny added a new feature for 2012 that has me really excited. It's called Publification. Like Yarny, it is free. It allows you to export directly from Yarny to Publification's online self-publishing site. Remember that feature of exporting snippet titles? Publification automatically recognizes them as chapter titles if you choose, saving you a big step in formatting. You can import cover art, apply for an ISBN, share links to your e-book through social media, and sell your book right from their site. And the best part is, you retain all rights to your material. Publification does not get any rights to your book. They simply collect a percentage of sales.
Both of these tools are great for the aspiring author. Together, they can be indispensable.