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(tm) 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
Moonlight works great on TVs too! Way better than Steam Link, which can't even display HDR correctly nor run games at framerates beyond 45. With Moonlight HDR works perfectly and I get a silky-smooth 120+ FPS @ 4K.
They fucking better not cause Moonlight is honestly the best way to stream your games to TVs and other devices. Steam Link sucks so badly. It can't even maintain a 60 FPS stream over 2.5G ethernet, let alone 120+.