I discovered this software while participating in NaNoWriMo a few years ago. It is almost too good to be true. It keeps track of your wordcount and scrambles your text for submission to the official NaNo site. More than that, it helps you write in scenes. It is a little more complex than Scrivener, but in some ways that is good. Right now I am using both. You can write in one, save the text as RTF, and paste it into the other. Each has advantages. It is, for example, harder to keep track of characters in Scrivener, but there is no corkboard for scene “cards” in yWriter5.
Ywriter5 is just for Windows, though. If you want to try it, the download link is below.