Syncthing, runs in background, synchronises everything between machines. Use it on my phone, tablet, NAS, desktop, Steam Deck.

This is not a very popular app, but I use it all the time. Full disclosure, this is my own app, but it’s free and open source.

It’s for transferring and managing files and folders over your local network. I use it whenever I need to just ad-hoc move files between things.

https://flathub.org/apps/com.sciactive.QuickDAV

Streamlink (with mpv) for watching Twitch streams with all the benefits of a real media player (like hardware video decoding) and without all the extra junk that occupies the Twitch web interface. It can also stream from YouTube and various other sites, record, and probably do a few more things that I haven't discovered yet.

Syncplay for watching movies with remote groups of friends, by synchronizing everyone's playback time, speed, and pause buttons. Rather than depending on a streaming service, it expects everyone to have a copy of the movie. I use it with Mumble voice chat so we can all converse while watching.

Draw.io is pretty great for diagrams.

Wiki.js can embed them too.

yt-dlp is my go-to for interacting with YouTube. Super helpful.

LibreOffice Draw can be used to modify PDFs, so I typically use it to fill out forms (whether they're "fillable" or not).

A lesser known one: QDirStat helps visualize the size of different folders on your PC. Great when trying to figure out how you managed to fill up that new 2TB drive in a couple months.

I use droposs.org to handle the family's games.

I have to give a shout to starship.

I'm in the terminal all day, and with all the context switching from interruptions and meetings, having all the terminal context in my face really helps me settle back into the groove.

Sure there are dozens of other ways to do this, but a little config for a hundred different tools, all of which is customizable, makes it a pretty easy choice for me.

I'll give a second shout for tig. Ncurses git interface with vim-like navigation makes exploring, staging, stashing, etc. super easy and way faster than the CLI. Certainly not a replacement, but a magical enhancement.

autokey — a recent "autohotkey" sort of thing for linux. It comes to mind since I recently had to find a replacement for the one I'd used previously which died of bitrot. Mostly I just use it for app-specific key remapping for Firefox so that I can disable its ^W which I only ever hit accidentally when it was possible.

I think Cherrytree is my most important app. It's primarily for making hierarchical lists but you can hyperlink between nodes and to external files and URLs and you can insert files, images and tables. I pretty much use it for organising my entire life and archiving important files, links and documents. The database is a single file (which you can have encrypted), so it's super portable and you can sync it between devices. You can easily theme it yourself too (the default theme/icons looks quite old school).

I also love GIMP 3.0

Unsure how well known it is, but flameshot (screenshot tool).

I prefer CLI usually, so: zoxide, the zsh git plugin for aliases (e.g.: gst is git status), fzf zsh plugin and the tldr command comes in handy sometimes.

Also, this might be useful just for me, but due to orientation of my living space, I have to fiddle with monitor brightness at least three times a day so I made myself a little Qt tray wrapper around ddcutil's ddcquery which can change standard vesa monitors brightness/contrast (DDC/CI communication).
There is also ddcui/gddccontrol GUI that does the same thing.

I use ncspot daily as a replacement for the official Spotify client.

Lazygit is my git TUI of choice.

Bonus: For note taking, I'm a huge fan of Neorg. It integrates well with my workflow, it's very intuitive and looks neat in the terminal.

I really like logseq for my personal knowledge base, notes, and in general magaging things from to-dos, reading lists or personal projects.

The way you can structure information relationally, and espetially the "referenced from" preview that's on each page makes it really easy to get an overwiev of something, and the query language to make your of previews (such as, list every unfinished to-do with a deadline this week) makes it a pretty powerfull tool.

Figuring out syncing, especially on mobile, took some time, since it lives in a git repo, but there are some plugins for it on PC and on mobile I just use Termux with CLI git.

I really like keyd for general key remapping.

Also occasionally obs-cmd if I'm streaming with obs-studio.

Localsend for easy file sharing between devices

I use Fortune a lot at the terminal, even though it's usefulness is questionable at best.