to name a few I like:

• Read You, brilliant RSS reader. Now syncs with Freshrss. Inspired by Reeder on iOS
• Voice, audiobook player. I use it in fact to listen to audio from youtube videos (downloaded with Newpipe). For audiobooks (and podcasts) I use
• Audiobookshelf (needs server)
• FUTO keyboard with decent on device voice recognition! (exists as a standalone)
• 8Vim, weird keyboard I really like
• Fairemail has been my trusty email client for years. Don't be spooked by its many options, once accounts configured the UI doesnt show unecessary stuff IMO. Some unique and useful features.
  But if you want something simpler there's K9, now Thunderbird. Although I still experience the issue with emails not arriving on time with this newer version
• KeepassDX, great password manager, good UI, keeps improving.

Thunderbird, Firefox, Element, Home Assistant, DAVx5, AntennaPod, Immich, NewPipe, Task.org, SyncThing, Tusky.

• Seal: Allows me to download YouTube videos, Instagram reels, etc. It's a simple and pretty front end for yt-dlp.
• Fossify Calendar: Finally kicked Google Calendar to the curb. No fuss calendar app.
• AVNC: Allows me to remotely control my laptop from my phone. Requires installation of VNC software on the laptop (I'm using TightVNC).
• Binary Eye: Barcode/QR code scanner with history.
• Loop Habit Tracker: Helps me keep track of habits – eg. going to the gym X times a week.
• Island: Allows me to create a clone of app. Useful for apps that don't support multi-accounts.
• Simple Time Tracker: I use it to keep track of how long I spend playing a game, since I like documenting the info for HowLongToBeat.com

Other than these, K9 Mail and NewPipe.

FUTO Keyboard

Thunder

Weather Master

Fennec

Thunderbird Android

Bitwarden

Material Files

Droid-ify

Mull, syncthing, k9, aegis, binaryeye, RoMote, Breezy Weather, newpipe, auto-auto-rotate, rethink, radiodroid, linksheet, missed notification reminder.

• Librera
• Obtainium
• AI Wallpaper Changer
• Aegis
• Mihon/Tachiyomi/whatever-the-current-fork-is
• Termux

All of the above managed through Obtainium, directly from the source.

• Bitwarden (Google Play link)
• Eternity for Lemmy (Google Play link)
• HordeNG (taken down from Google play, but if was basically just a wrapper for a web app which you can install easily from the browser anyway - https://horde-ng.org)
• Baby Journal (also a web app wrapper, available as a web app at https://baby-journal.app)
• probably others from Google Play I don't use that often and don't remember whether they are open source or not

Three of those are made by me, but I also use them often (that's why I created them in the first place).

Freebloks and Unciv for when I'm bored

TrackerControl for keeping my phone mostly ad and tracker free

Auxio for music, but I'm considering an alternative, I can't make it ignore certain folders so a lot of received Whatsapp audio shows up on the playlist

Vanilla Music – the music player I use.

I was struggling to remember which I used, and then I remembered that Firefox (or Fennec, to be precise) is open source.

Also, Jerboa. Hi.

HeliBoard is also pretty good. As is Tusky.

Kvaesitso and Heliboard are my most used, since its just the homescreen and keyboard lol. For favorite I'd probably say Pipepipe, been watching a lot of YouTube and I'm sick of ads.

It sucks that Syncthing is discontinuing its android app

It would be up near the top of my list.

1. Newpipe
2. Newpipe
3. Newpipe
4. Mull

Most used: k9 mail, Firefox, Voyager. Favorite: termux is awesome but I don't use it much.

Added: Trail Sense is another great app that I don't use all that much at the moment.

• Catima: stores barcodes of membership cards, etc.
• Aegis: two factor auth app with backup options
• LocalSend: open source cross platform airdrop clone

koreader, tracker control

Also, they're new but opentimelimit and unlockmaster for productivity

Olauncher

Some great apps on here, iv noted a lot of them down to try out

Eternity, Material Files, Nextcloud, Termux and idk if it counts, but Expo Go, it's amazing for development.

Mull, 1List, Notally, and Cheogram are pretty heavily used.