Emacs and org-mode
I am a long time fan of mind maps and outliners. Recently, I've spent a few hours messing around with one of the oldest text editors of all: Emacs. It goes back to the 1970s. Over time, the editor was "extended," so that it now has over 2,000 commands, and has "modes" that let you program in various software languages, read newsgroups, send email, create websites, and on and on. But the mode that interests me most is an outliner called org-mode (for "organization," I guess). In addition to powerful commands for creating outlines that can be expanded, collapsed, moved, and instantly exported (to pdf, html, odt, and more), org-mode also supports an almost overwhelming array of text editing and planning options (to dos, due dates, etc.).
Way back in 1982 I bought my first computer, a Kaypro II, running CP/M and a "Perfect" package of software. Perfect Writer was, it turns out, a subset of Emacs. So looking at it again, I find that my fingers remember things my brain had forgotten.
Back then, I left Perfect Writer for Wordstar, then NewWord. Most of what I wrote wound up being printed, and Perfect Writer had extremely complex and unpredictable formatting commands. But these days, I produce files, not pages, and Emacs, or emacs (more frequent spelling), is a fascinating, and even absorbing environment. More to the point, files created back in the 1970s can still be used today by almost any other editor. Why? Because emacs produces plain text files. The commands allow you to do almost anything, but the files themselves are readable by everything.
Moreover, emacs is fast on almost any computer, absolutely bulletproof (40 years of debugging), and to top it off, is free in both senses (open source and no charge). It runs on Unix, Linux, Windows, and OSX. There are even various applications for Android and iOS, although none that are quite full-featured.
Be prepared, though. On the one hand, you can pound out text in seconds. A handful of commands are all you need to get started. But emacs is one of the deepest rabbit holes in history, and some people now manage literally every aspect of their lives in it. There's a learning curve that's more like learning to play piano than learning to type. It is its own universe.
But when it comes to writing, about anything you want to do can indeed be done. You just have to dig it out first, then try to remember how you did it.
I started with this file, a good, basic introduction. Have fun!