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.

MSYS2 (Free, Windows) by MSYS2 Organization.
I feel this is the easiest and most straightforward way to get a dev environment running for basic stuff such as gcc, node, etc. Obviously there are better methods but this suffices. Caveat: no way comes close to to unix systems, MacOS and (obviously) Linux are far, far head than Windows to set-up a quick, lightweight software development environment. It's 2023 already, Microsoft!

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.

MacOS

iTerm2 (Free, MacOS) by George Nachman
Essential. iTerm2 is an excellent, feature rich terminal emulator for MacOS.

Ityscal (Free, MacOS) by Mowglii Apps
Small, cute calendar in your menu bar. Seriously good.

Little Snitch (45€, MacOS) by Obdev GmbH
Essential. This is a ridiculously powerful host-based application firewall for macOS. Fun fact: there are cases where a malware uninstalls itself if it finds Little Snitch installed in your Mac. Lol.

Jump Cut (Free, MacOS) by Steve Cook
Essential. Jumpcut is the macOS clipboard manager you need in your life.

BlockBlock (Free, MacOS) by Objective-See
Essential. BlockBlock monitors common persistence locations and alerts whenever a persistent component is added.

AppCleaner (Free, MacOS) by FreeMacSoft
Essential. AppCleaner thoroughly uninstalls apps.

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.