Bibisco is a "free and open source software for writing novels. With Bibisco you can organize chapters and scenes, manage revisions, export novel in pdf or rtf, and write with a fully featured text editor. You can create a novel structure, define premise, fabula, narrative strands and settings: geographic, temporal and social context. And, most of all, with Bibisco you can know everything about your characters" (OS: Windows, Linux)
CreaWriter is a Windows program designed to boost your productivity and creativity. Its full screen, minimalistic interface provides a distraction-free environment. Indulge yourself in a relaxed and calm atmosphere and achieve a whole new level of concentration!
Inspired by OmmWriter, a Mac OS X only application, CreaWriter allows you to customize both background image and ambient sound" (OS: Windows)
Quip is a "free, communal online office suite with word processing and spreadsheets. It shines as a collaborative tool—great for anyone doing some kind of group novel. Quip offers free apps for iOS and Android, so you get full online, mobile writing options. Quip stores all your files on its own servers" (OS: Windows, Mac, iPhone, iPad and Android devices)
Write Monkey is a "Windows zenware writing application with an extremely stripped down user interface, leaving you alone with your thoughts and your words. It is light, fast and free. With an array of innovative tools under the hood and full Markdown* support, it helps you write better" (OS: Windows)
All of these links were found while rifling through the articles at PC Mag.