Scrivener!
The frustrating thing is that, at least for me, there are no perfect word processors geared for novels and other scenarios where you manage large text masses.
Scrivener is one of those cases where you have a pretty excellent software that doesn't have a lot of problems OSS alternatives have. I have smooth time with it. But at the same time, the software always could be better.
Probably the best OSS novel writing software I've used is Org-Mode for Emacs. But, you know, it's based on Emacs, so it squeaks around the edges and gives the impression that it's a miracle it runs as brilliantly as it does.