For PC, the two tools that fit me best are yWriter and WriteItNow.
yWriter is up to version 5, it's free, and it's created by an active writer that also writes code, listens to people who provide him inputs (and donations) and he constantly improves the tool. It lets you get right into writing, or outline, or build background, or develop character traits, or storyboard, whichever way you work. No MAC version, but it does have a Linux version. And you can change
the language. I sent Simon a donation last year and as much as I use it, I probably owe him another one.
WriteItNow is another good writing/organizing tool. It has a few more tabs to park reference and background data, and even has several add-ons that bring catalogs of data about eras (Victorian, early US, etc) and personality types. I can see that this tool would be great if you knew you were building a multi-volume fantasy world where you needed a LOT of background data to keep your story straight. PC or MAC