Here is a growing list of open-source or (really) free softwares on various artistic domains and working on Linux, Mac, and Windows:
Vocamen has created this tool to allow you to easily create web and mobile pages (see in mockup) without code. https://html-builder.vocamen.com/dist/
Replaces Photoshop for direct digital paintings. Also it comes with useful plugins like G'mic (made in France!). Both vector graphics and raster brushes. You can search for additional brushes on Internet: https://krita.org/en/
Replaces Photoshop, except for its high-end plugins and functions. When you're used to Photoshop, you might find hard to adapt to GIMP so someone adapted GIMP to Photoshop keyboard shortcuts and feel making PhotoGIMP: https://github.com/Diolinux/PhotoGIMP
Our favorite 3D creation tool is also able to draw in 2D, using Grease Pencil. What is great is that your drawing is not in pixels but in vertices that you can sculpt, simply, etc. It's like vector graphics with high-end render. Le t me know if you'd like tutorials about this: https://www.blender.org/
2D vector graphics that can import and export SVG, PDF, PNG, ... Toolbars and menus are getting old but it's powerful: https://inkscape.org/
This one is not open-source, it's a webapp to quickly replace Photoshop for most of the needs. Even dynamic objects are present and it comes with a ton of templates. Very effective: https://www.photopea.com/
To make cartoons and webtoons. Another windows app working on Linux: http://firealpaca.com/
Of course Blender is #1 in this list. You can do 99% of what you can imagine if you take the time (years) to learn it. It has a steep learning curve, mostly because most of the tutorials miss how Blender works (python mixed database of vertices, hence the name: Blender). Philippe, Vocamen's director, made a short film applying footages on 3D, only using Blender, even movie edit. Let us know if you'd like tutorials about Blender and which subject: https://www.blender.org/
CAD oriented, parametric design. For engineers. Interface is hard, you need to switch workbenches to find missing tools in another workbench. It comes with a plugin library which is great: https://www.freecad.org/
Free interior design application that you can quickly use to get a 3D scene to import into Blender. Also comes with 1400 3D models (home furniture) from contributors: www.sweethome3d.com
For processing and converting meshes. Pretty hard to get at first. Can build a mesh from a cloud point (3D scan). You'll need tutorials for most of the functions though: https://www.meshlab.net/
To remesh an object as QUADS following creases. Comes into Blender as a plugin and also as a standalone app: https://github.com/wjakob/instant-meshes
Code to CAD software. Import and export STL, PNG. Build complex objects using library constructive-compiled, then import in Blender as STL. For code lovers: http://openscad.org/
Layers (onion skins) Animation tool used at Studio Ghibli. You can scan your drawings and color them digitaly. Also works with vector graphics (SVG). https://opentoonz.github.io/e/index.html
Pattern Making software for fashion designers: https://smart-pattern.com.ua/en/valentina/download/
NURBS, STEP, IGES 3D editor. The exact missing block of Blender. You can import OBJ and export to IGES. It's made for designers and 3D artists. 90 days evalation though so it's on the end of the list: http://moi3d.com/
This little tools is great for various reasons: You can remove scenes from a movie without destroying the film and without re-encoding it (tutorial needed?), you can re-encode and encapuslate to various codecs and formats, you can extract audio tracks: https://avidemux.org/download
Used to stream but also to record tutorials, or simply to make small screencasts: https://obsproject.com/
A middleware to tweak your webcam feed and stream as a virtual webcam to OBS Studio or else: https://webcamoid.github.io/
A free video Editor with hidden gems (right click to discover). 3D animations are from Blender: https://www.openshot.org/
Last because it's not open source, but not least and free with limited functions with the option to get a perpetual licence to unlock everything. From time to time, you can get a free licence when bying a device like the speed-editor. It's a professional editor, like FinalCut, Avid, Premiere and we encourage you to use the great documentation to learn it: https://www.blackmagicdesign.com/products/davinciresolve/
Free DAW when evaluating with a small countdown at start, no limit to evaluate it. I encourage to buy it though. FX filters are great, and you can add VST (even 64bits windows VST plugins on the Linux version). It can also import and export your videos so you can enhance your audio tracks: https://www.reaper.fm/
Open source scoring app. Beautiful inteface and pretty easy but less and less free. You'll like to add addtional instruments to produce masterpieces. You can also find musescore files (mscz) on the internet: https://musescore.com/
Unique non stop playing music with a CC-BY license. You can record some part to use as background music in your production: https://play.generative.fm/browse
The most listened artist in the World! ... because every youtuber used one of his CC-BY licensed music, at least once. Very good pieces: https://incompetech.com/
Made by a pastor! Smart app to track characters, events, arcs while writting chapters: http://www.theologeek.ch/manuskript/
Free Novel Writing Software as a PWA web app (works offline on your phone and tablet). Easy and deep functions like characters cards, snowflake diagram, planning board, grid planner, mind map, timeline, and personal challenge mode: https://wavemaker.cards/
For game makers. World-Building & writing tool: https://github.com/Elvanos/fantasia-archive
Publishing software for PDF, printing house, like InDesign: https://www.scribus.net/
I guess there's no need to cover the IDEs like VS Code, IntelliJ, Sublime, ...
Both hardware and software: You buy inexpensive boards shipping microcontrollers and shields to build unique IoT solutions and services. You can get very creative and make robots and animated art: https://www.arduino.cc/
Design and simulate logic circuits: https://sourceforge.net/projects/circuit/
I won't come into details about office tools, you might already know them : LIbreOffice, ... Here are less known but useful tools:
Mardown editor + thoughts organizer. Easy to learn and improve. After months, you realize that you need it every day : https://obsidian.md/
A mind map software to lay down all your ideas in your creative phase then organize them in your development phase: https://sourceforge.net/projects/freemind/
Ebook manager and converter. Interface is cumbersome, but once you know how to use it, it gets very useful. Also has plugins to convert even more: https://calibre-ebook.com
If you can't bare Mac or Windows anymore, you can now easily switch to Linux. I use Ubuntu-PopOS and all above softwares are running on it, some using wine, an emulator for windows applications. Everything is free and you don't get pushy updates, bloatwares and you're exposed to far less viruses: https://pop.system76.com/