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
I would say.. Termux/SSH clients, as they are text based... Tachiyomi, with disabled animations..
But beside that, not much. I just abuse the fact that e-ink devices that I do have, do refresh the screen at decent speed (e.g Hisense A9, Onyx boox Tab Ultra), so.. most apps are actually useable, even if... the experience on other devices would be pure misery.