Cross-Platform

Sublime Text by Sublime HQ
Sublime, indeed. Free to evaluate, $99 for a license. Available for all platforms.

Workflowy ($49/yr) by Mike Turitzin and Jesse Patel
Workflowy is a zoomable list that provides unprecedented flexibility in organizing your ideas. I use it all the time and I have grown fond of it.

Windows

(g)Vim (Free, Windows) by Bram Moolenaar, Christian Brabandt and others.
The hard learning curve is worth it. Pretty much the swiss army knife of editors. Available on every platform but I particularly like it on Windows. What? You want my vim.rc?

Sumatra PDF (Free, Windows) by Krzysztof Kowalczyk, Simon Bünzli and others.
Super fast, lightweight and just nice. Only pdf reader I have used that comes close with Apple's excellent Preview.app.

Bulk Crap Uninstaller (Free, Windows) by Marcin Szeniak and other contributers.
Thoroughly uninstall apps. Robust feature set. Recovery options. Cleans up leftover registry files! Highly recommended. I lied, it's essential.

Homebrew is essential and my preferred package management system on MacOS.

I also use these: Dropbox, iAWriter, Preview.app, Apple Music, Things 3, Apple Keynote, Overleaf, Google Docs, Obsidian, VLC, iMovie, DaisyDisk, Apple Podcasts, MalwareBytes (free), Amphetamine, Anki, Apple Notes, Google Chrome & Firefox.