I recently stumbled across this superb little word processor, and I’m just blown away by how good it for being made by one dude for free. It’s like a slimmed down version of Scrivener or Papyrus, with a wonderfully simple and easy to use interface.
It’s open-source, and works on all platforms (Windows, Mac, Linux, including phones and tablets). This lady here does an excellent overview of its features.
If you’re looking for a nice little app for writing, outlining, or planning something, I’d recommend giving it a look.
My new favourite answer for everything like this is Asciidoc. You can incorporate latex or mathjax directly into docs.
I do like text formats, but even when a graphical editor for it exists, they usually don’t provide the convenience of being able to edit equations in-place. Usually there’s just raw LaTeX editing, or some kind of pop-out editor.