Scrivener is worth every penny. In fact, it's the only piece of software I recommend students buy. Everything else can be done open-source. For this post, I'll merely list why I use Scrivener, and leave more detailed instructions for later posts.
I use Scrivener because it encourages you to:
Scrivener is a very deep tool and can do lots of other things well too. But these are the biggies for me. Details in later posts.