As in title, preferably open-source, but doesn't have to,
I will start:
KOReader - Amazing reader app, and being able to use the same app across various devices, even non-android ones, is pure bliss. Japanese support is pretty good too, which makes me happy. Though.. Sadly no vertical text support :c
Syncthing-fork - Being able to keep a library of books, and some files, and easily keep the shared folder across various devices, is mega comfy.
Tachiyomi - Cute manga reader.
NewPipe - Really pleasant youtube client, that i sometimes use to listen to things in the background.
Kaku - An useful Japanese OCR app, that works.. quite decently for when I encounter kanji that I do not recognize, and is not text.
EinkBro - A web browser, that on e-ink devices, is quite comfy. Has few... quirks, but works well on e-ink
YouTube ReVanced - YouTube with adblocking, SponsorBlock, and Return YouTube Dislike built-in. There is also ReVanced for YT Music.
NetGuard - Firewall that doesn't need root. Works on system apps too so you can help curb the amount of spying done by system apps and apps that don't need an internet connection to function.
Connect - Most functional Lemmy app I could find (and I literally tried them all). Still not as smooth of an experience as Relay, but /u/DBrady has announced no plans to port it to Lemmy so Connect will have to do.
Stremio - Any show or movie you want from any streaming platform (Netflix, Hulu/D+, HBO, Apple TV, etc.), all in one place. The app runs on TVs too. Requires plugins to work, but costs as little as $3/mo or even free, depending on what plugins you use. You'll never need to subscribe to any other streaming platform.
Splitwise - Easily split bills with your SO/roommate(s). Stop arguing over expenses.
Google Rewards - Answer surveys, get Google Play credit. Paid apps are now free!
I've also tried all the Lemmy apps I could find, and must say I'm still the most fond of Voyager. Almost every single time I launch, there's a new update, and new features are showing up extremely fast. Now that there's customizable swipe gestures for all use cases, it's become so awesome to use. I even had an issue with comments getting deleted if I swiped too far while composing, opened a ticket on GitHub, and dev had it fixed within a day. They've just started submitting app versions to to both the Apple and Android store, if you don't want to use the web app.
I second Google Rewards, I also haven't paid for an app for quite some time now. A small note - if it asks if you've visited a specific store recently, the reward is a lot bigger if you say yes, say you bought something, and how you paid. Even if you didn't 😁 There's always (at least) one plausible option, a store you were near, and then a few that look to be fillers. I personally only pick the plausible ones, just in case it's some sort of test. They also rolled out uploading receipts where I'm located recently, and must admit I don't mind Google knowing what I purchased at whatever supermarket.
My problem with Voyager is that it's a web app. It's extremely laggy/unresponsive and the keyboard doesn't work. Firefox doesn't play nice with web apps and I refuse to use any other mobile browser cause I need UBlock Origin and NoScript.
I've been using it with Firefox and haven't had any issues with lag or speed on a regular pixel 6. I'm curious what issues you've had or if you've used the most recent versions?
I'm typing this from the new sync, because it's more familiar to me and I prefer it's ui.
I've been using Voyager as a web app on Firefox since I left Reddit in the blackouts and had zero issues with it. I've only moved over to Sync today because it's the app I used for Reddit