I'm presently using a free trial version of Scrivener to write my NaNoWriMo "novel". I know that I come off sounding like a walking advertisement, but it's the best word processing for writing. They offered a free offer to NaNoWriMo users and that offer is open to everyone.There will be another free one month offer in December. The software will be available in early 2011.
For the download Beta version for Mac and Windows
"... if you've been struggling with getting your manuscript organized (or under way), I strongly urge you to give Scrivener a whirl. (And no, this is not a paid endorsement!)"
—Mark Sarvas ( Harry Revisited)