I've started using this to write a couple of books I'm working on and thoroughly recommend it to anyone with large amounts of writing to do. It is remarkably intuitive and natural to use, and remarkably powerful as a means of organising thoughts, keeping notes, incorporating texts and much much more, as well as providing neat distraction-free modes for actual writing. It's not open source but pricing is very reasonable, especially if you are a student or academic - less than $40 - and it is available for Mac and Windows.

It's primarily a tool to support the writing process, not for finished drafts. It can be used to generate pretty decent simple-ish output, but the idea is to export the results to a word-processor or desktop publisher to do final tweaks.

The only big problem I have with it at all is that it doesn't neatly integrate with reference managers, reflecting its origins as a tool for authors of fiction, novels, screenplays etc. Sure, you can insert relevant codes from things like EndNote, Papers or Zotero, then format bibliographies etc when you export the document, but it's clunky and unintuitive, and not at all friendly or flexible. I'm really hoping that an update with such support arrives soon as this is going to be a real pain as I go on. On the other hand, it has great annotation and reference tools that can be pulled in to do part of that job, so it is not a complete showstopper, but it's a major omission.