Hard to answer but maybe Haiku or GNU Emacs

Linux, Tor, and the Ballistica game engine/BombSquad game (not fully open source as stuff used for sensitive data remains closed source 😔)

Edit: forgot git lol

OpenSCAD and Gitlab. I can quickly iterate on designs through code, push it to my Gitlab instance, and have my CI/CD pipelines pick it up, render it, and automatically slice it in some common profiles to send to Octoprint

For games:

• Anuto TD (found a few days ago, isn't super feature rich but still fun to kill time)

• Mindustry (never played a game like it before, ended up supporting by buying it on Steam)

• Supertuxkart (I love how many custom add-on karts and tracks I have)

For non-games:

• Termux (allows me to get apk files and install Revancify for add free yt)

• VLC (I don't mind slow updates and have yet to switch mostly because I can't find anything better that isn't more complicated than it needs to be and/or is closed source)

• KDE Connect (I have almost always had problems with moving files from and to my desktop via cord)

I'd include something like Linux, but I personally feel that's kinda cheating because of how large it is compared to the others.

Neovim. It's an awesome editor and it has a great community and ecosystem.

https://www.kicad.org/

Currently OBS and Motrix

Definitely OpenFOAM. It competes with commercial software that costs thousands of dollars.

NetHack!

Firefox, Neovim, Pass (password store) and Wezterm. I heavily use all four of them.

I also need to give a special mention to Aegis Authenticator on Android.

Uptime Kuma is a fantastic selfhosted status page system. You can use it to track and notify you of network outages or it can scrape a url for a key word and alert you when it's found. I've heard people using the keyword feature to find out when RPI go back in stock for example.

I use it at work to keep track of our systems and their uptime as well as cloud systems we use.

Right now, it's Warpinator. Makes at-home wireless file transfers so damn SIMPLE.

VS Code