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
moonlight-android - opensource android application to connect to your NVIDIA gamestream functionality provided by most modern NVIDIA graphic cards
I use it often for streaming my computer from my bed to play tactic/mouse heavy games, but latency wise it’s very good if using hardware encoding. Less friction than a parsec and/or steam link setup.
Additionally (though I haven’t tried it) there is a server software to emulate the NVIDIA™ gamestream functionality for AMD/Intel/etc, giving you an open source and low friction way to setup remote game streaming and/or desktop use https://github.com/LizardByte/Sunshine
Sunshine is amazing. I can vouch for it. It’s in active development and works really well.