Asklemmy

Qgis, Firefox! Comes top of mind

QGIS and OpenStreetMap for mapping

Currently OBS and Motrix

https://www.kicad.org/

vim

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

I'm getting a lot of use from Syncplay recently

Hard to answer but maybe Haiku or GNU Emacs

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.

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

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.

Suricata

uhh probably uhh AOSP and calyx os

paperless-ngx

Bulk Crap Uninstaller

Media Player Classic (I'm unsure if the latest iterations are or even if the Home Cinema edition is open source), TOR, qbittorrent, firefox, thinderbird, obs to name a few that I use regularly.

I have used a lot of stuff over the years but my favorite would have to be a little command line program called cowsay. It takes whatever text you feed it and puts it in a speech bubble above a cow, hence the name.

Helix text editor.