I flip flop between all kinds of things, but at the moment I'm using the always awesome Atlantis Ocean Mind. Small footprint, only a couple of MB download, cheap to buy, has brilliant typewriter sounds and can output directly to ePub with multi-level TOC, has an overlong sentence feature and loads of other goodies
If you're on Windows this is the no1 word processor in my opinion.
http://www.atlantiswordprocessor.com/en/*
*I am not affiliated in any way with Atlantis, I just think they rule!!!