As it stands, Substance shows great promise, but is not yet usable for daily work. It is one of the most stylish authoring systems I've seen to date, and it makes writing fun. It is free, and you can install it on your own server for maximum privacy. Once its creators iron out the kinks, it might offer a lovely way to write online.
Note: The Download button takes you to the vendor's site, where you can use this Web-based software.