micro: Really good TUI-based code editor with actual sensible keybinds. Very intuitive, I wish it was more popular but all the hype is on vim-like editors
geany: Lightweight IDE, I use it as my personal code editor. It's really good once you enable the file browser plugin and ignore the projects feature. LSP support coming soon
kate: Basically the QT alternative to geany, also a great code editor and has LSP support already for a long while, but not so great if you are on a GTK desktop because the UI will look very different
ffmpeg: It's decently popular but it should be even more popular. Our lives would be harder directly or indirectly if ffmpeg didn't exist
gimp: It's very popular in the linux and unix world, but it's underrated in the sense that everyone for some reason says the interface is atrocious etc and many people also consider gimp to be very weak or bad. Even when i was using windows years ago I ditched photoshop for
gimp and I was not hindered at all and didn't think the interface was bad, and now that gimp has non-destructive editing it's an absolute beast. You can do very complex and beautiful image manipulation on gimp. It does require skill of course as you don't have content-aware features and AI features, but if you are experienced with advanced image manipulation then you will make amazing things on gimp as you would with photoshop or krita
qemu: Almost everyone on linux uses virt-manager for running virtual machines, but I think more people should give a try to the barebones qemu CLI. Yeah the help screen could have been better and yeah the defaults are very minimal to the point where it doesn't specify memory use, audio, etc. But with a reasonable command you can get a virtual machine up and running with a cpu setup, memory size, graphical backend, usb, audio, networking, etc.
kdenlive and shotcut: 2 great video editors and kdenlive is especially comparable to premiere for a lot of things. Need VFX and compositing? You can use blender
ncdu: TUI application, great for analyzing the disk usage of a directory