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!