I love the concept of this writing site, Writer: the internet typewriter. The simplicity and the color scheme (which can be changed) frees me to imagine. And I like the sound of the manual typewriter. Throwback, I know. (Here's a similar one, DarkCopy.)
Another new one which I've just discovered is Draft. You might try it out as well.
These let you concentrate on content, not formatting. Just what the Muse ordered.