Adding on:
Inkscape - vector graphics program
Meshrom - photogrammetry
Handbrake - video transcoding
MakeMKV - rips DVDs and Blu Ray into video files
7zip - file compression and decompression
Droid48 - Truly excellent HP48 emulator for android
LibreOffice - free word processor & office suite (not without some recent drama though, I guess)
I’m sure I’m forgetting plenty, but hey, more for additional commenters to name.
Edit: Removed Audacity, apparently I’d missed privatization drama around that one too
I definitely considered FFmpeg (I mean, it does everything, and pretty much as fast as possible), but the sense I had was that people were mostly posting about tools that were reasonably accessible to novice users, with nice-ish interfaces. FFmpeg is pretty daunting to newcomers.
OpenSCAD (CAD, but with a programming language-style interface) is kind of in a similar category. It’s pretty powerful, and for someone who thinks like a programmer it can be relatively easy to learn, but if you don’t already understand 3d transformations on a pretty intuitive level, the program doesn’t have a lot of features to ease you into that.