The fish shell, makes using the shell feel almost painless.
Hammerspoon, I use it to automatically switch audio devices based on context, so every call uses the best microphone currently available, window management via keystrokes, limiting media keys to Spotify, tons of other things. Indispensable.
Arduino, I don't think I would be able to tinker with microcontrollers as much if I had to write C and use obscure toolchains directly.
Solvespace, a limited but usable free CAD for simple parts to be 3d printed. Wish there was a real contender to the commercial ones though, or a free tier that doesn't smell like it's going away any time. Still very thankful that SolveSpace exists.
The Scala 3 compiler and the VSCode plugin for v3. Absolutely love the language and the experience is so much better than with IntelliJ, haven't had as much fun writing code in ages.
This will be a bit controversial, but Kubernetes, because if people use it via GKE, EKS etc. then I won't have to learn their organically grown solution to the same dozen-or-so operations problems, and I have yet to see one that isn't a hot mess in some way or other. Also anything running on top of Kubernetes won't be built the very old-fashioned vi-edits-on-server way, great for sanity.
Various modern messenger apps (Telegram, Whatsapp, Instagram's direct messages, ...) because I would hear a lot less from some highly cherished and very non-technical people in my life without these incredibly slick and fun and convenient apps. As one who's old enough to remember being dependent on landlines and payphones and letters, this still feels like a miracle.