I made it to Linux! What is your must-have FOSS or Free Software for linux?

Here's some creative software that replace the functionalities of Adobe software & more.
photo editing: GIMP
vector images: Inkscape
drawing/painting: Krita (GIMP also fine for this)
video editing: kdenlive
3d modelling, animating, etc.: Blender
audio editing: Tenacity (Audacity fork made after the buyout without telemetry)
DAW: LMMS
media player: VLC or mpv
if there's any other specific software you're looking for a FOSS alternative to, don't hesitate to ask. You always have more options on Linux than you'd think.

BONUS: here's some command-line toys that are not useful or necessary, but are just real fun to take a look at especially if you're new to linux:
cmatrix (does the matrix code rain thing)
cowsay (ascii cow with speech bubble)
sl (steam locomotive in your terminal)
cbonsai (generate bonsai tree)
neofetch (this one is actually very useful and will print system information in an aesthetically pleasing way)
- wow, neofetch is really cool! thank you!
  
  It's mandatory for online boasting.
- You forgot the package hollywood.
- fastfetch is neofetch in C instead of bash, makes it much faster.
- Thanks for this!
- Cmatrix,SL and neofetch work on android too through termux, I bet others would too, but I have only tested these two

KeePassXC (there's a Firefox extension too) and Syncthing are the first things I add to a new install.

Another nice-to-have is KDE Connect, it connects your desktop with your phone to sync notifications, send files, control media playback, use as remote input, share clipboard, send commands, and more
- I'm not sure it will work with cinnamon.
  
  I use it in Cinnamon.
  
  It should work normally, there's even cinnamon applets for controlling it from the panel

Calibre - eBook manager/reader
Gparted - disk tool
Keepass - password manager
VLC - the greatest video/music player
Waydroid - run android apps

Welcome! Some of my must-have FOSS software for GNU/Linux are:
ONLYOFFICE: Similar functionality to that of MS Office, but free and open-source, very nice compatibility with .docx documents and all the excel formulas I use are still there.
Boxes: If you like or need virtual machines, Boxes is one of the best FOSS solutions out there, I have made Windows, BSD and Linux virtual machines using Boxes and they work flawlessly, and the drag-drop feature to send files from the host to the guest machines is absolutely nice.
Konversation: In my opinion the best graphical IRC client, with HexChat also worth noting.
Kdenlive: I have used many video editors in my life, both FOSS and proprietary, but Kdenlive is the one who made me stay. I have even remastered old 80s Betamax videos using only Kdenlive.
TeXstudio: If you like LaTeX, this editor is absolutely wonderful and it works out of the box.
Prism Launcher: If you like Minecraft, this is the only launcher that actually worked on my Fedora installation, and it's so easy to install mods, resource packs, shaders, etc. that I already consider it to be the best FOSS launcher for both premium and non-premium instances.
HandBrake: I just love this open-source video transcoder so much.
fre:ac: I have used this FOSS audio encoder since I was a kid when I wanted to convert mp3 music to a format that my DSi could read. Nowadays I still use it to convert from and to any type of audio and it just never fails.
RaccoonLock: A modern-looking and private password manager that is wonderful if you just want to store your passwords locally in your PC and you do not care about syncing them with other devices (although such feature is partially possible through the creation of backups).
It's also worth mentioning other FOSS software like VLC, VS Code (though it's not entirely FOSS, with Codium being an actual FOSS version), OBS Studio, GParted, PDF Mix Tool and FreeTube. Welcome to the GNU/Linux world! I hope you enjoy it and you find these utilities useful :).

random suggestion but do you play guitar? take a look at guitarix if so, you won't be disappointed

MPV, although you could also have used it on windows already. Also freetube, you can also combine those two with a little bit of internet search.

Logseq is a pretty nice FOSS alternative to Obsidian I came across recently. Tmux is absolutely necessary for any terminal work. Wezterm is my favorite terminal emulator because you can easily disable all of the shortcuts except very few you want (tmux handles most of terminal stuff for me). Some new alternatives to old command line utilities:
bat is cat but 1000 times better. I love it so much!

fd is better find
rg (ripgrep) is better grep
zoxide is better cd
dust is better du
- You know any better cd?
  
  I use zoxide plus fzf which ends looking like this.
  My default go-to for a better cd was teleport when I still was on bash. The tp command can be aliased to cd. I don't think it will run on other shells though.
  
  What? It's literary in my comment.
- Personally, instead of Tmux, I'd recommend trying Zellij. I started with Zellij right out of the gate, but a friend of mine who dislikes Tmux a lot quite enjoyed Zellij, so I assume it's somewhat better.
  
  I know about Zellij, but in its current state, I think it doesn't make sense for me to switch. I really lobe my Tmux setup, and I don't think I would be able to replicate it on Zellij, mainly because of Tmux plugins I use. I also don't think that Zellij currently has any advantages over Tmux except for better default config and user friendliness that I don't need. I'm looking forward to the Zellij ecosystem getting better tho. Some day, I will probably make the switch. The WASM tabs feature looks extremely promising!

I personally recommend LibreWolf over Firefox. It is a fork of Firefox, but it includes some additional settings for better privacy.
Flameshot is a pretty useful screenshot tool that functions similar to the Snip tool on Windows.
If you're going to be installing apps via Flatpak, I recently learned of an app called Warehouse that allows you to view all the Flatpak apps you installed, the user data associated with each app, and their file location.
If you want another option for LibreOffice, you can try OnlyOffice, but I personally prefer LibreOffice.
If you're looking for a text editor that's like Notepad++, I recommend checking out NotepadQQ.
Finally, if you want a notebook app similar to Microsoft OneNote, I'd like to recommend Joplin.
- For Flatpak apps, along with Warehouse, Flatseal allows you to view and edit permissions for each app, which is not only useful but sometimes mandatory when an app has misconfigured permissions
  
  Oh yes, Flatseal is also a good tool to have! Thanks for adding that!

Bottles makes using Wine real simple.

Flatpak for all *possible installs of user apps. But if there's an app I respect most, it has to be MPV.
- Aside from the fact that I've used it long enough without encountering any breaking issues or bugs, it is very powerful and highly customizable.

KiCAD for PCB design.
PulseView for USB logic analyzers.
DSRemote for remote control of Rigol oscilloscopes.
FreeCAD for 3D CAD.
SDR++ and SDRangel for SDR.
Fldigi, wsjtx & QSSTV for ham radio digital modes.
- I haven touched my hackRF for years. Got to give SDRangel a try.

For me it's (outside of the usual stack):
https://github.com/sezanzeb/input-remapper (Input-Remapper, if you have annoying proprietary thing you want to re-bind this is an amazing way to handle that)
This project made Linux viable for me, and I have not looked back at all.

helix (vim like text editor)
kate (kde text editor)
dolphin (kde file manager)
supertuxcart (most modern linux game)
- I would recommend Xonotic Its like unreal tournament and its fantastic. Smooth as butter no lags.

Blender: If you're interested in 3D modeling or Animation
Inkscape: If you have any need for a vector graphics program thats a bit like Adobe Illustrator
OBS: If you need to do any screen recording or livestreaming
Haruna Video Player: It plays videos and can also play youtube videos if you paste in a link. (This also pulls in yt-dlp as a dependency, which allows you to download youtube videos and the like from a terminal)
btop: A nice looking system resource monitor that runs in a terminal

Inkscape for drawing vector images.
Loads of command line tools to process PDF files, like extracting images or text.
All those tools for automated processing of data, including script languages like perl.
The MediaWiki engine that allows me to run a local wiki at home for my hobby.

syncthing - Sync files across internet, works very well
netdata - Very comprehensive monitoring software for servers
Firefox
wine - Without it (including proton) I couldn't make the switch, it's kind of a necessary evil but it's not wine that's the evil.
KDE Desktop - My personal preference, I used Ubuntu and Pop OS, gnome doesn't suit me.

I always start with Syncthing, which is a cross-platform p2p syncing client I use to share documents between devices
- KDE connect?
  
  I never tried it seriously mostly because I don't need all of the features it provides. But yeah that can be an alternative too

I'm surprises no one has mentioned the ones I use most days.
git (version control software development)
openssh (for ssh connections to other devices)
handbrake (video transcoding)
Element (matrix client)
- @Strit @blakeus12 forgot about element, but its sadly quite a terrible client on Linux. although that could be the fault of my nvidia graphics
- I literally had to run the windows version of handbrake in virtual box because for some reason the linux version can't save to the same directory as the input files while batch processing.
  basically the same otherwise, fine for single files.
  
  I've had no issues saving to same folder as source when doing batch transcoding queues on Arch Linux. As long as the input and output files does not have the same filename it's fine.

I always keep GIMP and VLC player installed. If you get comfortable using command line. Tmux and Neovim are a great duo.
- thank you

Helix editor

LibreSprite is cool for creating and animating 2d pixel stuff.

Welcome on board!
You revealed in your previous post to be a gamer. Therefore, I'd like to focus on software that might help with that (in alpabetical order):
Bazzite-Arch
Bottles
Conty
Lutris^[1]
For a one-stop-solution for all your problems related to package X not being available in the repos of distro Y; consider the more than excellent Distrobox.
You should probably start with this one as the others might be less intuitive to you at the moment. Furthermore, their use-cases and thus why one might prefer the others over Lutris in the first place might not be clear currently and not even be stuff you worry about in the first place.

ncdu
- for a bit more context, ncdu is a Disk Usage analyzer that runs in the terminal. If you've ever used WizTree on windows, its kinda like that. Really useful to see whats taking up space on your disk

vim !!
- Neovim too!

Nextcloud

Welcome! As for the basics: Bleachbit (system cleaner) Flatseal (Flatpak permission managed) Elisa (music player) GTKHash (GUI way to verify checksums) FreeTube (YouTube front end) Okular (PDF viewer & editor)

ProtonUp-Qt is an easy way to install and manage different Proton versions for gaming.

Xmoto
Supertuxkart
a development environment to learn programming

Depends what device you run, but Xournal++ is useful. Otherwise pdftk.
- Xournal++ is for drawing over pdfs like MS OneNote

I'm loving these suggestions, definitely saving the post for later!

Gimp and PGA

- appreciate it!
  edit: LibreWolf is awesome! TYSM!!!!

Komikku is my go to manga reading app and I honestly can't use anything else now.

Install everything as a flatpak, it is the future. Anything else will be outdated or can lead to headache (dependency issues).
Here's a simple GUI video downloader.
Here are elegant, easy to use apps that cover many different needs.
EDIT: Tell me where I'm wrong