A bit more niche, is Weasis - Dicom Browser for medical images. Alternative is also ImageJ which is used a lot in for scans too.

Wikipedia. Not an app but still deserves a mention.

I still can’t get used to calling programs apps

Librewolf, FFmpeg, Vim, Wine

OBS, and Blender. Two industry shaping software solutions that ere fully open source and free.

The Dialer.

• Comes with every phone
• 10+ digit number instantly connects you with millions of people, services, and institutions
• 3 digits connects you with life-saving emergency support
• Very low-latency voice support
• High quality audio (most of the time)
• No ads
• No obnoxious UI

All kidding aside, I’m routinely astounded at how we have yet to top the ease and utility of old-fashioned phone service.

uBlock Origin leading the pack by at least a furlong.

KiCad. GNU Linux. Blender. Gqrx. Rclone. Syncthing

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.

7 zip, VLC, Paint.net, proxmox, home assistant

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.

Signal. Highly secure communication. No ads. Easy to use.

YouTube clients like NewPipe

LocalSend, Immich, Signal, Aurora store, Radio Garden, Gray Jay, yt-dlp, and Bitwarden just to name a few

Linux