I can share files, links and clipboard items between my phone and my computer. Use my phone as remote control while watching stuff. I see phone notifications on my computer and don't miss calls when my phone is in silent mode while I'm at my PC. It has made my life so much smoother.
Everyone knows about VLC but I don't think many people know it has a really good Android app! I use it as my primary music app and it's great. Even has android auto support and a surprisingly sleek interface.
I’ve recently discovered Organic Maps that allows offline viewing of open street maps. I’ve been using it since maps.me has completely gone down the drain with premium subscriptions and paid download limits (for the same open street maps data lol).
SyncThing - to sync my files between devices and avoid the big corporations cloud (use it for notes, Keepass database, photos, etc)
Logseq - super advanced note taking with tagging and relationships between notes (all store locally)
Authpass - opening my Keepass password database on my phone
GrapheneOS Camera - just to avoid the Google camera app as I have Google Photos disabled on my phone and needrd a good app that doesn't break when going to view the gallery (since it tries to open Google Photos)
Simple Gallery - to view my photos on my phone
Signal - I have it but I have very few friends that use it unfortunately
AntennaPod - for podcasts, I'm thinking of self hosting a podcast tracker to sync my listening habits across my devices, we'll see if that happens
Vinyl Music Player - to play my local mp3 files and playlists (I use MusicBee to manage what I sync to the phone as my mp3 library is quite large, and SyncThing to actually copy stuff over)
Not really an app, but I'm going to add https://kagi.com/ here; it's time for a shakeup in the search industry that actually works, and Kagi delivers.
Controversial, but Telegram is such a solid open source app in terms of UI/UX and a good middle ground between something like Discord and Signal.
Now that I've shared my unpopular opinions...
Standard Notes is a really solid secure note taking app.
All of the Proton AG apps and services (Proton Drive, Proton Calendar, Proton VPN, Proton Mail, SimpleLogin).
Bitwarden.
"Privacy" (while not FOSS) is a pretty great software for using unique billing information per site (which helps with fraud protection, tracking, etc).
ZeroTier is awesome for remote access (everyone seems to recommend TailScale these days, but ZeroTier is very much of the "do one thing and do it well" mindset, they're also the "OG" FOSS encrypted VLAN solution).
Kopia I've been really liking for backups (great features include: the ability to clone a cloud repository of backs to a hard drive or different cloud via the software itself, a GUI, a simple CLI interface, and configurable policies to keep track of your preferences so you don't have to)
Rednotebook for journalling and Tomb to easily encrypt it through the command line
Librewolf because manually making tweaks to Firefox was kind of tiring
On my phone
PokerTH because I wanted to learn to play Texas Hold 'em without micro transactions and a required online connection
AntennaPod because I dislike using Spotify for podcasts
Aegis because it was easier to transfer authenticator codes. I think Google Authenticator now allows for local backups and exports
Edit: Actually the coolest (but least useful) has to be brow.sh. I'll attach an image but essentially it lets you run a browser through your terminal in a way that's a little more indepth than apps like Lynx.
In order to really see the extent of what it can do you really need to see how it handles video playback.
Everything search app is easily my most used on my Windows PC. Instant, as-you-type file search by name with detail sorting and wildcard support. I set a keyboard shortcut as well. It puts Explorer search to shame. As good as Spotlight, which I was missing from when I had a Mac in 2008.
LaunchyQT - modern fork of Launchy that actually gets development. App launcher, dead simple.
FanControl - Fan curve software with all the features I want including hybrid temp sources, sensible automatic settings, visual graphs with an intuitive interface.
AutoHotKey - Hotkey scripting language that I can use even as someone who barely codes.
Transcribe! - Not open-source but it's a reasonably priced audio and video slow down app for transcribing.
Where Signal is secure - Threema is private and secure (e2e encrypted, uses PFS but doesn't need your phone number for sign ups. You can be 100% anonymous should you wish)
I cycle to work. This takes like 60-70 minutes, there is a ferry ride. The app MoopMoop is like google maps and a weather app in one. It show the rain on your route.
I'm using Aves Libre as my gallery app in my phone, i really like it, works pretty well and looks pretty clean. It can classify the media by the apps (It need to access to the app list to do this).
To watch some videos i use Libretube, but it's a shame that (i think) there's no invidious related apps in android.
And the maps that i use are from OpenStreetMaps, OsmAnd in my phone. Works pretty well despite not using google services that it is pretty good in my opinion. I also use it to add some notes or interest points and then back phone contribute with that i wrote.
FSearch [Fast searcher of all files on my computer. Like Search Everything for Windows but worse in many ways]
AudioBookshelf [Podcast server]
I use all of those pretty regularly. Honorable mention on iOS is a program called Is It Snappy? which helps me measure input lag. It doesn't collect any data or run ads (rare trait on a phone app). I actually made a purchasing decision with the help of this thing to correctly conclude that the Nintendo Switch Pro Controller had a noticeable input delay (enough to make me return it). The funny thing about that was if I just looked up spreadsheets others have done I would have seen that same conclusion there, too, without having to go through the effort of buying it myself :P.
I forget what it's called, but I have one I got off F-Droid that just does a spectrogram of the microphone input. It's cool seeing the world yet another way.
I use Tachiyomi (J2K fork) for reading manga and manhwa, and Shosetsu for fanfiction (AO3 extension). MoeList for MAL (My Anime List), Myne for public domain books, and Librera for EBUPs (I like using TTS). Also NewPipe (SponsorBlock fork) is my only YouTube client.
I’ve recently discovered Organic Maps that allows offline viewing of open street maps. I’ve been using it since maps.me has completely gone down the drain with premium subscriptions and paid download limits (for the same open street maps data lol).