uBlock origin

Firefox. I hate how inflexible other browser are.

Bitwarden. Otherwise I won’t be able to log on to any of my accounts.

The kernel. I can take or leave most things, but I'm not going back to the days of writing directly into memory-mapped registers.

Android. As bad as it is, if I had to use iOS or Linux phones it would be even worse, at least with the current state of Linux phones.

But actually, maybe if Android didn't exist, the FOSS community would focus more on Linux phones and they would be an actually good option. Maybe Android shouldn't exist?

You can pry Vim from my cold dead hands.

On Android, it's probably a little utility software called Quick Cursor (it's not FOSS). It's incredibly convenient being able to spawn a cursor on your phone from thin air that you can use to reach the "unreachable" portions of your screen, especially if you are holding your phone with one hand. Besides being a "phone touchpad" it has a bunch of ways of triggering actions/shortcuts, for example: volume or brightness control, launching an app (I use it for launching a floating calculator, notes...), opening notification shade, copying text (it can copy any text that is under the cursor, even if it's not selectable)...

It's not that I couldn't go without it, but it changed the way I use my phone and it would feel really weird without it. It feels like it should be a part of the OS.

Linux, seriously, it's in my phone, my router, my desktop, my ISP and nearly the entire infrastructure of the internet upon which I rely uses it.

Firefox, uBlock Origin, uBlacklist KDE, Dolphin, Kate, LibreOffice, CherryTree Kid3, Flacon, LosslesCut, qBittorrent, VLC Musicolet, Simplenote, F-Droid, AuroraStore

A compiler. I mean, yeah, I guess I could go back to writing asm, but I really don’t want to.

Going back to a "normal" text editor after using Vim for a few years would be horrible

Life without qBittorrent would also be pretty difficult, hell no, I'm not paying for DRM content that requires proprietary software to watch

vmlinuz

firefox

Yt-dlp. It's basically the only way I download music nowadays.

Python

7zip

KeepassXC + KeepassDX

Leisure Suit Larry

I'm bored so I'm just going to make a list:

• Lightroom Classic (I've tried Darktable, just not for me. I take a lot of photos on my DSLR and I've been using Lightroom since 2015 so for me it's worth eating the awful monthly subscription that I split with someone else.)

• Anki (flashcard app, very popular among med school students and folks trying to learn new languages. Open source and tons of useful decks available. I've aced plenty of exams thanks to Anki.)

• Bitwarden (finally caved and got a password manager-- could not be happier)

• CHIRP (the best for programming handheld, mobile and base station radios)

• CrystalDiskInfo (great for checking the health of SSDs and HDDs)

• DaVinci Resolve (love using this for video editing-- pirated copy was easy to find)

• Deluge (great for torrenting)

• foobar2000 (I love it for music)

• Greenshot (useful screencapture software)

• inSSIDer (great for wifi analysis)

• IrfanView (very good for photo management)

• MusicBrainz Picard (amaaaaaaaaazing god tier music management software to get all the correct metadata/album art)

• reWASD ($7 but it's so good for no BS macro'ing of keyboard/mouse/gamepad shortcuts and profiles. I have two PCs and two mice + gamepad attached to my PC and this software is very helpful. I think the license is for life.)

• WizTree (SSD/HDD visualization tool that is useful for figuring out what's taking up too much space on your drive)

git, vim/nvim

Obsidian.

Since the Internet in general is getting harder to find genuine information, it is becoming increasingly important to save anything important to you. One day it could just disappear without warning. Obsidian can be used for an offline knowledge base. Design it however you like. I do recommend NOT watching YouTube Obsidian “gurus”, their system works for them not you.

Notepad++

Vi/Vim - had it on every computer I've owned or used since about 1991.

qBitorrent, undoubtably

Termux on Android.

I've got some videos on my phone I might want to watch on random computers, so I serve them up with NGINX. I've got wget-created mirrors of some old websites on my phone, so I serve them up with NGINX. Other files I may want to move out from my phone to untrusted computers on the network can too be served up simply by NGINX.
I've got the full Wikipedia zim file from Kiwix on my Micro SD card, so I run kiwix-serve (behind NGINX).
I've got all the music on my phone, naturally the phone is then running my Navidrome server (behind NGINX).
Of course, I may want to manage this from a computer, so it's running SSH server.
My phone is always connected to VPN and uses NextDNS, naturally I may want to use this with other computers, but I can't install software to computers I don't own (I mean, I can, but ... it would be disliked), naturally it is then running Tiniproxy HTTP proxy server.
Some desktop GUI apps can be useful on a phone too. noaa-apt, Kid3, Audacity, desktop Firefox, Handbrake because I am too dumb for ffmpeg, so I run XFCE DE on it. Naturally, I can access it from a computer (I know) too, after all it's accessed via a VNC server.
Am I stupid enough to expose something using HTTP protocol running on my phone to the internet? Of course I am! I can use cloudflared.
Do I want to encrypt a file? I can use GPG.
Do I want to create a compressed archive? I've got TAr and GZip.
Do I want to browse Gopher? I've got Lynx.
SSH or telnet somewhere? The clients are there.

LiGNUx, VLC, Firefox w/Ublock, KDE Connect, Dolphin, Kate, KDE. Vim, i3wm, Keepasses, yt-dlp, deluge, freecad, librecad, slic3r/cura. Some of these are clearly redundant or overlap. My use cases vary

Non-foss: Steam library.

I wouldn't spend so much time on the PC if I had to pay a premium for every little thing much like I've experienced with my arts-related hobbies.

Probably an OS

On Windows: EarTrumpet

Being able to quickly change audio outputs is awesome, I am always bouncing between headset and speakers. Also the pop up volume mixer is better than the built in one. Been using ET for years and years, can pry it from my cold dead hands.

bash

Joplin App

Windows: PowerToys. First thing I get approval to install on a work machine. PowerToys Run (Launchy on roids) saves me from the built in Windows search, a quick calculator, etc. PowerRename gets used more frequently than I care to admit. Video Conference mute is a second nature key combination. Can't remember the name of the window manager module but it is a key part of my workflow.

Android: As I've mentioned in a reply, Edge Gestures has been on my phone for years (first installed on a Pixel 3). Having 10+ apps accessible (especially 2FA, password vault, home assistant) from any screen, plus gestures for quick controls (flashlight, brightness slider) is incredibly handy. And unlike the notification shade, the edges of the phone can actually be reached with your thumb.

Linux: Docker. It's been an instrumental part of building out my home server which allowed me to kill my Microsoft 365 & Google One subscriptions. For me it has been the gateway drug in to learning more and more about self hosting - to Proxmox, LXCs & VMs, pihole and unbound, etc.

libreoffice, particularly calc. I keep all my finances and planning in spreadsheets I migrated from excel years ago.

Obsidian

Firefox, emacs, restic backup, bitwarden, linux/bsd

My first instinct was to say GIMP or Firefox, but I could still use Krita or Chromium in those cases.

I'd say Anki then. I don't know of any other FOSS flashcard app this good, and I have so much saved on it that losing it would be devastating.