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
NetGuard Pro - Allows you to see and control traffic from all apps, so you can prevent data flows to 3rd party hosts like the ones from Google or Facebook. The pro edition is paid and necessary, but it’s all open source, just not gratis
Aegis Authenticator - open source 2FA authenticator
LibreTube - alternative, privacy-respecting, open source YouTube frontend using the Piped API
Not quite wrapping my head around aegis, what’s the point? It seems like a password manager, more or less
Is netguard still maintained? I seem to recall seeing the developer freeze development maybe a year or two ago? Hopefully I’m wrong.
