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Scrivener is a writing program for Mac OS X and Windows. It is also in beta for Linux. It has a long history of sponsoring NaNoWriMo and Script Frenzy. It costs $40 USD, but usually has deals for NaNo participants (20% off) and winners (50% off).
The program has many different writing features, such as scriptwriting, a place to store research items, and the signature corkboard. Keywords and labels further assist in organization. For Wrimos, the target word counts are a must. Backups are automatic and there are many export and compile options to tweak drafts.