A bit more niche, is Weasis - Dicom Browser for medical images. Alternative is also ImageJ which is used a lot in for scans too.
Signal. Highly secure communication. No ads. Easy to use.
7 zip, VLC, Paint.net, proxmox, home assistant
KiCad. GNU Linux. Blender. Gqrx. Rclone. Syncthing
Kodi
Lithium EPUB reader
NAPS2. I go paperless as much as possible, but still have to scan stuff sometimes. It’s the GOAT for scanning.
systemd
I still can’t get used to calling programs apps
Librewolf, FFmpeg, Vim, Wine
YouTube clients like NewPipe
Home Assistant, not only an App but it changed the way i look at IoT/Smarthome and in that way it brings me a lot of comfort.
Organic Maps. After switching to graphene, I quickly found plenty of apps replacing the “defaults” I had on stock android, however, a good app for maps was impossible to find until I stumbled over that one. Great UI, local maps, even has a navigation feature. Completely replaces google maps for me.
LocalSend, Immich, Signal, Aurora store, Radio Garden, Gray Jay, yt-dlp, and Bitwarden just to name a few
Wikipedia. Not an app but still deserves a mention.