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
Sadly, oftentimes, Forums are replaced by discord, despite... how different those are.
And, discord is inferior in so many ways. Not only you can't easily search for the content, you also need an account on centralized proprietary software, that also is quite resource heavy. Not to mention the privacy concerns.