Bitwarden is one I use several times a day. They do have a support plan for like $10 a year that gives a couple extra features like TOTP support, but the base level is incredibly robust. It’s open source, too. I know a lot of folks also host their own servers with Vaultwarden, but that’s a little beyond my skill level.
Firefox, ppsspp, termux, VLC, Tachiyomi(SY), and KeypassXC/DX are coming to my mind. Probably there are a lot more. These are for android. Although they do apply to desktop except termux and Tachiyomi.
Edit: I haven't added the various FOSS tools as they don't really come in "App" Category. Some of them:
Weawow is a completly (also ad-)free weather forecast app run basically solo by a Japanese guy. I was surprised when I found this app that it was so good in every aspect that I had to donate the guy. It has has more than half a Mio. reviews on google play with an average of 4.9 . Idk of any free app with that many reviews having this kind of rating, well deserved.
Taking the opportunity to get on my soapbox and remind everyone that free software still requires someone's time and effort to maintain. If you've been using a free app for a while and you and you enjoy it (and you have the means to do so), consider sending a donation to the developers/maintainers! It's a good way to help ensure that the great, free app you enjoy stays great and free.
OpenStreetMap is a free, editable map of the world, created and maintained by millions of volunteers. It includes data about roads, buildings, shops, points of interest, and more.
Many of the benefits of Google Maps without all its spying and advertising.
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.
Krita. I don't use it at a professional level so I don't know if it's missing important features, but as far as I know it's also used by skilled artists. Also, the documentation is great.
Others have mentioned most of my favorite tools. One thing I'd like to add is SageMath. It's a mathematical software that's comparable/better than commercial offerings like Mathematica and MatLab. I've rarely seen anyone in the academia using anything else these days. If someone does use something else, it's just because they're more used to it. SageMath is by far the best tool for most things math.
Also, while typing about Sage, I was reminded of how great of a tool LaTeX is. If you want to write anything that'll be more than a single page, LaTeX is probably the best way to do it.
I can't believe Photopea (https://www.photopea.com/) is free. It has nearly all the same features as Photoshop and works in just the same way. But it runs in the browser! Super quick, regularly updated, and free. Amazing.
Just had to give a shout out to Stallman & GNU. I've seen a lot of mentions of thanks to Linux on here, but Richard will never let us forget that Linux ain't shit w/o GNU software to interact with it.
Just think of the number of GNU programs you've used, just in a typical day on the terminal.
OsmAnd. Navigated me through numerous countries on holiday. Found us places to eat. So useful it persuaded me to start updating the map locally to help any fellow travelers
Joplin notes. Use this every day without fail
Nextcloud. Self hosted cloud that Ive come to rely on
DaVinci Resolve was the last app to really surprise me. It's a fantastic app for video editing with a ton of functionality. Most of the paid functions seem like composite fuctions of the free functions or overly professional tools, but for getting started with simple 2D- or 3D-animations or short film editing, it's beyond amazing.
Limiting myself to free as in freedom (no ads, not free to use because you are the product):
KeePass/KeePassXC, GnuCash, Firefox, LibreOffice, digiKam, GIMP.
For both professional and personal use, I can list 2 that you likely haven't heard of:
https://github.com/meowtec/Imagine
Imagine PNG/JPEG optimization - it basically compresses images and photos so you can email lots of stuff back and forth without using the likes of WeTransfer.
https://ditto-cp.sourceforge.io/
Ditto Clipboard Manager - a multi clipboard for Windows. Ever try to paste something, only to realise you've already copied over it? Its use and helpfulness is so ubiquitous, I just could never live without it anymore.
I don't know if this will show up or is already in the list, but: Rufus. I burn all my thumb drives for os installs with Rufus. It also lets me bypass a lot of the windows garbage that they've tracked on to the installer, like making you sign in to a Microsoft account to install. Also, Ventoy. It's a multiple OS installer, so one big thumb drive lets me install any number of OSes from it.
While I'm setting up those OSes, ninite gets me my windows programs, and Snappy Driver Installer Origin gets me my drivers. No more laptops with pre-installed bloat for me!
As a Mechanic. Ampol Netlube, from lawn mowers and passenger vehicle to motorcycles and heavy mining/industrial equipment, I can find how much lubricant it take, what viscosity and specifications.
ShareX. The ability to screenshot or record a video of practically anything onscreen with any shape or form, assign hotkeys to certain tasks, and the ability to automate all of that and attach to other applications/processes for a smoother workflow? For free? Count me in.
Bit of a different answer, but I enjoy Daylio. Very simple and easy to track your mood and activities to look back on. Not really super useful, but very interesting and fun!
Here's how you actually free space on your computer in a way that matters without installing some malware "fix-it" program or need a computer divining rod to find every random file:
WizTree: Scan your entire computer hards drive(s) in a matter of seconds and display a very useful graph and data about where your space it being taken up. It's eons faster and easier to use than the leading competitor WinDirStat to the point where I can't imagine why anyone would use something that isnt WizTree.
BCUninstaller: It helps uninstall as many apps and programs off of your computer automatically with little to no user interaction needed beyond hitting the "start" button
BleachBit: It deletes all the temporary and nonessential stuff that gets accumulated over time. It won't clear as much as BCUninstaller or deleting stuff with WizTree and a lot of apps will generate most temporary files again anyways but I do typically see a decrease of around a gigabyte or two. Worth a shot in any case.
Winget: While not a software in the general sense, Winget is a package manager built into Windows 10/11 itself that lets you automatically download, configure, and install a ton of programs in one command via command prompt or PowerShell.
Every single program I've listed here are available on Winget.
Definitely R/R Studio if you're in data analysis. Runs circles around commercial Software in terms of scope and customization.
A little harder to get into of course, but once you get the hang of it, there's little you can't get done with it.
BOINC. If you're a scientist, you can use it to distribute massive computational workloads for free. And there are tons of computing volunteers who will gladly do the computation for you. If you love science, it's a great way to engage with some cutting edge research projects and know that you're "doing my part!". You can help research cancer, develop new open source drugs, map the galaxy, or just do some fun math stuff. Just install and pick your projects, no PhD required! There was even a projects for a while to reverse all the minecraft seeds that some people participated in. https://sopuli.xyz/c/boinc
Libby, the ebook reader app that is synced with my library card! It works quite well, and though I technically pay for my public library via taxes, the app is free and fantastic!
Youtube hands down. Youtube is the best dad, best teacher, biggest information hub, and arguably the best meme generator in existence. The fact i can dive into any type of video content and come out feeling like i gained so much is incredible.
Taking the opportunity to get on my soapbox and remind everyone that free software still requires someone's time and effort to maintain. If you've been using a free app for a while and you and you enjoy it (and you have the means to do so), consider sending a donation to the developers/maintainers! It's a good way to help ensure that the great, free app you enjoy stays great and free.
DMDE. I know they have free and paid version. Even with the free version I was able to recover 500gb of data 100% with all the folder structure intact. With the free version you will have to manually select each directories but it was worth the effort.
Also Mega. I have multiple legacy accounts that has permanent 50gb space.
For me, Reaper without a doubt. I've been doing audio work for the better part of a decade now it's become my main DAW of choice for the last 5 years. Technically a license costs you $60, but every feature is available in the "trial" version that never expires. SWS script support and the insane amount community skins/extensions makes it well worth the $60 if you plan on buying. I've replaced many ProTools with Reaper setups.....
tachij2k, i use it for all my comic book, manga, and hentai needs. easy to use. access to a bunch of websites. I can download for offline uses. Amazing app.
QGIS (https://qgis.org/en/site/). A Free and Open Source Geographic Information System. There are also free basemaps available - I use the same basemaps as I use at work in ArcGIS Pro.