Additive Manufacturing
FreeCAD Linkstage - RealThunder's fork of the FOSS CAD package is less buggy, has improved rendering, and is much easier to use.
PrusaSlicer - A snappy alternative to Cura for slicing 3D models for printing. A lot of awesome features and it's constantly under development.
Blender - I've done a little here and there with Blender, but Cycles works great for product renders. It's such a vast and amazing program that can accommodate so many different use-cases.
Music Production
LMMS - An FL Studio-like DAW with a simplified workflow and robust features. Lackluster plug-in support out of the box, but the addition of a VST host and waveform editor make it a fully-featured way to make music.
Element - Fully open-source VST host with support for VST3. Also works as a standalone application, which means you can create plug-in chains without touching your DAW. You can also save presets of those chains, and do crazy signal routing with the two-dimensional geometry nodes-esque UI.
Vital/Vitalium - It's literally FOSS Serum. You can follow Serum tutorials, and have them turn out. A wavetable synth that's so darn easy to use, you'll never want to use anything else. This is the quintessential FOSS future bass producer's synth.
Dexed - DX7 cartridge manager and emulator. It sounds like an awesome 80s FM synth; what can I say? Must-have for synthwave and noodling around with new sounds.
Sforzando/SFZ - An open standard and a free player for said open standard. Allows for what are essentially lossless, unzipped soundfonts.
VSCO/VSCL - A few decent symphonic instrument libraries based around SFZ. Both are CC0.
Freepats - A decent place to find more SFZ instruments. A few classics like a dry Tele and a few CC0 pianos live here.
Audacity- The only FOSS waveform editor worth using. It's extremely flexible, has a ton of useful built-in effects, and makes for a great companion to LMMS when you need to make more in-depth edits to samples.
Cardinal - FOSS fork of VCV as a VST, which enables you to create crazy virtual eurorack creations and play them with MIDI. You can also use it standalone, and the sheer number of built-in plug-ins basically guarantees your dream of automatic music generating machines are only a few clicks away.
MusicGen - A recent ML tool by Facebook that can be run locally; essentially SOTA on few-shot text-to-waveform music generation. If you have a somewhat-high-end GPU, it will probably work for you. A great tool for sampling into weird ambient tracks.
RVC - A recent tool that is fast to train and provides extremely realistic voice-to-voice conversion, especially for vocals. Ever see those AI SpongeBob singing memes? This is probably how they did it.
Photo Editing/Design
PhotoGIMP - While I'm still using Photoshop, PhotoGIMP is an add-on for GIMP that attempts to port the Photoshop UI to... GIMP. It's mildly successful, and potentially can ease the pains of transitioning to a new program. I'm honestly too lazy to switch at this point, but it looked promising when I peeked the last time.
Inkscape - I suck at vector anything, but this program proved to be useful on occasion. I believe it's a serious competitor to Illustrator if you bother to learn how to use it properly.
A1111's Web UI - Now totally FOSS, this absolutely insane piece of software integrates with so many different useful plug-ins to accomplish basically any conceivable image generation or AI-with-images task imaginable. You can literally do anything from normal text-to-image generation to upscaling or colorizing, and even img2img; it's multi-modal to no end.
EDA/PCB Design
KiCAD - Hands down the best EDA package I've used. Granted, it's the only one I've used. Still, this is how FOSS software for engineering purposes should be designed. I wish they would send their UX people over to help FreeCAD out. If you need to design a PCB for anything at all, use KiCAD, period.
Programming
NodeJS - The sole reason JavaScript is worth learning for more general computing tasks; with the sheer variety of packages on NPM, it feels like you can do anything.
VSCodium - All of what makes VSCode worth using, and none of the creepy MS telemetry.
General Computing
7zip - The one program to conquer all archive formats. It works, and it's absolutely tiny. I've even installed this on Windows 2000, and of course it worked fine.
LibreOffice - Occasionally buggy, but certainly the best FOSS office package currently available. LibreOffice Writer and Calc are especially usable and work great.
VLC - Is there anything this traffic cone can't play? Superb video and audio codec compatibility, although it won't play a MIDI unless you feed FluidSynth a soundfont to atone for your sins.
Strawberry - For when you want to listen to tons of music, but you hate the clunky nature of other audio managers. Strawberry basically doesn't use a DB, and instead edits metadata directly. It will also instantly update when you add new songs or change metadata, so you rarely have to restart it. It's the fastest way to manage tons of music I've found.
PCPartPicker - A website, but still worth mentioning. This is basically the only tolerable way to part out a PC, and it makes sharing specs of your recent projects trivial.
Rufus - Someone else mentioned this one, but it's basically the only tolerable way to create bootable installation media. Works well, and it's FOSS.
Operating Systems
Manjaro KDE - The closest you can get to SteamOS's desktop mode. Based on Arch, like SteamOS, and the same DE as SteamOS.
ZorinOS - Tolerable derivative of Ubuntu LTS, especially for Windows natives.
Games/Emulators
Quadrapassel - Best Linux Tetris clone ever conceived. It's in my Steam Deck library, for Pete's sake.
Yuzu - Pairs well with a PC handheld and a "screw Nintendo" attitude. The Switch emulator that is often marginally faster (and often slightly less accurate than) Ryujinx.
OpenRCT2 - RCT, especially the first two games by Chris Sawyer, are some of the best tycoon games ever created. OpenRCT2 is a faithful reimplantation that is going places.