Here's a few I enjoy using that I haven't seen mentioned yet:

• Phonograph Plus: elegant music player using Material UI design
• Etar +ICS² : Etar is a nice-looking calendar app that uses Material UI design, and ICS² enables me to sync the Holiday Calendars provided on Thunderbird.net.
• Etar+ DAVx² + NextCloud : enables syncing my local Etar calendar(s) with any NextCloud instance (using CalDAV). DAVx² also allows for syncing my Android contacts to NextCloud (using CardDAV)
• AnySoft Keyboard: extremely customizable, varied keyboard layouts, for those who want all punctuation marks and quote signs built-in.
• Irregular Expressions: alternative keyboard for goofing around with unicode textstyles (🅒🅘🅡🅒🅛🅔🅢, 𝕺𝖑𝖉 𝕰𝖓𝖌𝖑𝖎𝖘𝖍 𝕭𝖔𝖑𝖉, Ｅｘｔｒａ ｗｉｄｅ, etc.)
• Simple Notes : minimalist "note to self" memos.

megladon for mastodon

Fennec, Bitwarden, Droid-ify, and Tutanota are my essentials I need for life.

termux - Terminal and shell, with Linux package environment so you can install and use packages like openssh.

Authenticator Pro. It's like Aegis but supports WearOS devices, so I can get my TOTP codes on my watch instead of having to get my phone out.

FairEmail. One of the best email clients I've used, across any platform. Highly customizable.

Syncthing for syncing photos to my home server.

Readrops for RSS - connects to my FreshRSS server.

Firefox, of course.

• Thunder: Lemmy App
• Scrambled Exif: Removes metadata when sharing images
• Feeder: Rss Reader
• KISS Launcher: Home Launcher
• NewPipe: PeerTube and YouTube app.
• Omni Notes: Notetaking
• StreetComplete: Easy, fun, and addicting way to make OpenStreetMap updates on the go
• OsmAnd~: Maps and navigation. Works phenomenally when in the backcountry and uses cell towers to locate you when gps alone can't.
• Tower Collector: Contribute BTS towers for the above.
• VLC: Video player
• WiFi Analyzer: Useful for improving and setting up WiFi networks.
• UntrackMe: Transforms links into their OSS alternatives.

I use a few apps from the SimpleMobileTools suite. They aren't full FOSS, they have basic and pro versions where the basic version is GPL3 and the proprietary extended features cost a few bucks. The basic FOSS tools are still decent, if barebones.

The suite includes:

• a calculator
• a phone dialer
• a music player
• a calendar
• a photo gallery (with basic editor)
• an audio recorder
• a flashlight
• a clock
• an app launcher
• an SMS messenger
• a camera
• a keyboard
• a note taker
• a file manager
• a contact book
• a simple painting canvas

I use the gallery and file manager the most. Though admittedly I threw a couple bucks their way for the proprietary extensions. It's not FOSS, but if it was going to be proprietary, I think it's one of the fairest deals in software these days. Better than another bloody subscription model, or holding the ad-free experience hostage behind a paywall.