Technology

Aard 2 - translator that takes you to wikidictionary
andOTP - offline OTP with easy option to encrypt your backup with your PGP key
AnyMemo - very nice flashcard app. Creating your own (with Tatoeba for example) is very easy
Carnet - simple notes app
FairMail - great mail app. IMO better than K-9
Feeder - RSS watcher
Ghost Commander - file manager. UI is a little bit clunky but much better than enything else
KeePassDX - password manager
Librera Reader - IMO the best PDF/epub/mobi reader there is
monocles translator - nice translator similar to DeepL
NewPipe - youtube and bandcamp
Ning - local network scanner
OCR
Odyssey Music Player
OpenCamera
OpenKeychain - PGP
OsmAnd~ - navigation and maps in general
CV Project - Mozilla projecton capturing voice recordings
QKSMS - for sms
RCX - cloud client
Shelter - the "work profile" manager
Simple Calendar - calendar with very nice widgets
Sky Map - you can point your phone in the direction of a star and see it annotated
Weather Widget - for the weather
WebMediaShare/WebApps - discontinued unfortunately, but it's great at appifying an URL, so you don't have to use official app

I use more, but these are the ones I think might be most interesting to others. I don't know if all of these are absolutely 100% open source (and that depends on definition) but all of these are in F-droid.

Bitwarden or KeePass as open source password managers. KeyPass is entirely local, unless you sync your password database on the cloud, and Bitwarden is cloud based but with the option to self host the server (I recommend Vaultwarden, it's lighter and written in Rust).

Joplin for note taking, especially if you use Markdown.

KDE connect for sharing files quickly between desktop and mobile - it's better than the proprietary fast share protocols I've tried.

Termux, for shell access and running Linux distros, albeit heavily limited.

Fritter as a Twitter client alternative, though I'm already avoiding Twitter for the most part.

Here are mine

Antennapod

Batterybot pro (for charge/discharge notifs)

Simple calendar

Droid-ify

Element

Feeder

Fennec

Simple files

Simple gallery

Jitsi

KeepassDX

Librespeed

LocalMonero

molly (fdroid Signal fork)

NewPipe

Simple notes

Ntfy

Odysee

Osmand+

ProtonMail and ProtonVPN (IzzyOnDroid repo)

Quickweather

Simple recorder

RedReader (reddit client) (dev is considering porting to lemmy)

Session

Termux

Tor browser

Tusky

Unstoppable Wallet

Forgot one, Tachiyomi, if you read manga.

I wanna piggy back off this thread if that's okay. What image gallery do y'all use?

I've been using aves or simple gallery for a while but I've never felt quite satisfied with them

Neo store - F-droid frontend

Sympathy - music player

Libretube - Youtube frontend that proxies through Piped

Material files - file manager

Aegis authenticator - TOTP 2FA app

Bitwarden - frontend for the FOSS bitwarden password manager

Magisk - The only non spyware root app

Cloudstream: Piracy movie streaming that supports multiple sites

Aegis - TOTP (alternative to Google Authenticator)

There is a bunch of apps on F-droid (and also playstore) which start with "simple..." all opensource. My list: simple calendar pro, simple contacts pro, simple dailer, simple draw pro, simple filemanager pro, simple gallary pro, simple notes pro, simple SMS messenger. No ads on any of them, and free. I have a total of 39 f-droid (so opensource) apps on my phone... So start with F-droid

I don't really use Android much now that I daily drive a Linux phone, but when I do these are my go-to apps:

F-Droid: FOSS app repository/"store" where I get most of my FOSS apps.

Aurora Store: FOSS app that interfaces with Play Store, allows downloads without account or tracking

NewPipe: FOSS YouTube alternative with the ability to download videos

Fennec: Fork of Firefox in F-Droid

VLC: Media player that plays almost anything

Hacker's Keyboard: My long-standing favorite on-screen keyboard of any platform

Terminal Emulator: Before I got into Linux phone, for running Linux distro chroots

I'll try to list a few I haven't seen mentionned elsewhere

Glider: Hacker news viewer GitNex: gitea client Scrambled exif: scramble photo metadata before sharing/uploading it Saber: handwritten notes, works pretty well with my android tablet

If you use a Nextcloud server, there's a good collection of apps (some official, some third-party) that work with it. The ones I use:

Nextcloud - main app, does authentication, file access, optional auto-upload photos Nextcloud Notes - kind of like Google Keep, but simpler. (IIRC Carnet is more like Keep, and also open) Nextcloud Talk - instant messaging, supposedly can do voice but I've never used it for that Nextcloud News - RSS reader that syncs your feeds and read/unread through your Nextcloud server

Plus these apps that aren't Nextcloud-specific, but work with it and other sync methods:

OpenTasks - ToDo list (needs Dav5x to sync) DAVx5 - Syncs contacts, calendars and to-do items between any CalDav, CardDav or WebDav servers and your Android system, so you can access them with any local contacts or calendar app. (For instance: K-9 Mail can use contacts from my Google account and my Nextcloud account, and Simple Calendar can do the same with my calendars.) Floccus - Bookmarks manager that can sync across multiple desktop browsers and the mobile app, using any of several sync options including Nextcloud

I'm a big fan of KOReader https://koreader.rocks

It is an eBook reader. Works brilliantly on eInk devices and regular phones/tablets.

Vimusic is awesome. It's YouTube music without paying or ads.

Quillnote is great for taking notes.

Metro as an offline music player.

Loop Habit Tracker pairs nicely with Atomic Habits or any other foray into productivity.

Stellar stat-tracking and usability. It's twice as great when you look at the alternatives and see mostly month-to-month subscriptions for similar features

FitoTrack is what I use for fitness tracking. It's pretty great.

Other than the stuff that's already posted -

1. Kvaesitso - Neat launcher. I was using Nova every since the days of the Nexus S. Switched to Kvaesitso when Nova sold out.
2. Local Send - Easy way to share files on the same local network.
3. Tailscale - Mesh VPN which can help you connect devices on different networks. I use this to bypass my ISP's CGNAT and access my personal media server remotely.