Supposedly Scrivener is the writing software to use if you’re on a Mac. But I’m not.
I use something called Page Four. I could just use Word, or any text editor but I really like the way I can break my project into chapters and scenes with Page Four. It allows you to add as many folders and pages/files as you want.
I’ve set my novel up so I’ve got one folder for outlines, world building notes, etc, and then a folder for each chapter. If I want to move a scene from one chapter to another, it’s a simple drag and drop – then I just have to do a little editing on the transitions. It’s come in very handy when I want to restructure a story without getting lost.
It’s a fairly simple editor, without too many bells and whistles, but how many times do you use all the crazy stuff Word can do anyway? It saves files in RTF format and they can be exported as one file into Word if you want, when you get to your final draft. It will also archive versions and you can take snapshots if you want.
You can try it out for free (limited by number of pages), and it’s relatively inexpensive. (Note – I have no affiliation. I just bought a copy awhile ago and really like it.)