What are your Android hidden gem apps

Syncthing.
Allows you to use a laptop and sync your phone to it. No iCloud or Google drive needed. All syncs locally.
- Wanted to also suggest Resilio Sync if you need something a little less fussy or smaller learning curve. I use it for many thins between devices, I never really "got" Syncthing altho I support their mission and excellent work the community benefits greatly from.
  
  You have used both. How does Resilio compare to Syncthing?

AntennaPod. It's easily the best podcast player I've tried, it's open source too which is nice.
- I wish it had a self hostable sync feature
  
  It does! Gpodder2go! It's super easy to host, too
  https://github.com/oxtyped/gpodder2go
- Every time I have tried a different app because I wanted some feature have realized I don't use the feature and end crawling back...

Aegis is a FOSS, local only 2 factor TOTP generator. Essentially a Google authenticator without the Google.
- I feel like KeePass is a preferable portable format that you can do everything any TOTP app can do without being tied into non-standard apps. Like .kdbx is a standard that works in any KeePass app
  
  I'd be concerned about storing both authentication factors in the same location, maybe if you kept passwords in one db and passwords in another. Not sure how well mobile apps support multiple dbs though
- Local to the Device? Google already did that to me. Then my phone died and I had the world's worst time trying to convince some of my vendors that I really was me. Like, I had to get my ID notarized in person and sent that paperwork off by international post in one example.
  I wouldn't want to go through that drama again. I moved to Authy, they keep my tokens encrypted on a cloud service. I could potentially be convinced to move to something self hosted, but never local-only again.
  
  Aegis can do automatic encrypted backups to a cloud provider or locally.
  
  That's why you back up your shit.
- Authenticator Pro works with my Samsung Wear OS watch. I get the codes on my watch instead of switching between apps on my phone to get codes.
- Aegis is what I have used for 2fa for quite some time now, but I think I should also mention Authenticator Pro . The feature set is similar to Aegis and, it's got a design that some people may find attractive.
  P.S: It also has Wear OS support, so that you can see 2fa codes from your watch (I can't tell how well this feature works as I don't own any wearable devices)

KDE Connect (FREE, open source) is definitely a gem. I love using my phone as a remote mouse and keyboard for my HTPC, and syncing clipboard, files, links, and notifications with the other devices I've paired with. (BTW, despite the name, you don't need KDE - or Linux even - to use it. It works on Windows and Android too.)
URLCheck (FREE, open source) is fantastic. It's a little pop-up that appears when you click a link, showing you the full URL and letting you modify it before you open with your browser/associated app (e.g., to remove tracking parameters):
URLCheck acts as an amazingly customizable and powerful intermediary when opening url links, allowing, among other things: to remove trackers, affiliate links, unnecessary elements, check Hosts, facilitating link holding and sharing, protecting against certain phishing techniques and many more...
- +2 for KDE Connect, the integration is amazing. I've used it on KDE and Gnome (gsconnect), all works very well
- URLcheck is a very useful app. I use it all the time.
- I switched to Device Connect because there's an issue with KDE Connect & mconnect (since a few months ago) where it re-pairs every time the phone loses network connection. DC has far fewer features, but its reliable and doesn't require running a KDE or Gnome service stack.

So I did NewPipe for using YouTube and that's all I have to say on the matter 😘
- Smart tube for Android TV.
  
  Would it be ok for you to do that as a top-level comment as well?
  Edit: Thanks!
- Grayjay has completely replaced NewPipe for me because it automatically updates my subscriptions

Smart tube for Android TV.
- [Excellent]

Ever work on electronics?
I use an apk called "Electrodoc" and it identifies and helps you figure out what all you need for all sorts of transistors and capacitors and smd's and inductors and all that sorts of crap. Has loads of info and pictures and calculations you can punch in for stuff. Super handy.
- thx, just downloaded
  
  Cool. Really gives good info repairing boards.

KISS Launcher. Fast and flexible homescreen replacement that puts quick search first, letting you optionally scroll through an app list.
- Niagara Launcher for me. Love the minimalistic approach!
  
  https://files.catbox.moe/9fm7c6.png
- I switched to it one year ago from nova launcher, after getting used to the change, I don't miss nova at all.

Aves Libre - Image Viewer
Just tried it 30mins ago. Super slick and feature packed. Available on Fdroid too.

Genius Scan. A shameful exception in my otherwise fully FOSS phone.
It scans multi-page documents with the camera, OCRs them, then uploads them automatically to Nextcloud (or manually to any other app, like Paperless).

Not sure how "hidden" these are, but they can be really useful
"AMdroid" by Smart Alarm Clock Team (Higley-configurable alarm clock with different alarm profiles, options for a post-alarm confirmation, and dismiss challenges)
"Engineering Converter Plus" by thermofluids.net (Has every unit conversion I've ever needed without needing to be online - Free version avaliable)
"Network Analyzer" by Jiri Techet (Good for helping to find the best spot/s for a wireless router or access point)
"Oldschool Editor : Text Editor" by AbhishekPandey (A no nonsense text editor)
"Unseen - No Last Seen" by Firehawk (Good to help look through message spam without alerting the spammer with a read receipt that your account is active)
- Aha, don't take it too seriously/literally, the point is to share apps that might not be super well-known or blockbuster-status that are nevertheless useful and hopefully purchaseable. I should edit the description.

Tachiyomi is a brilliant manga reader and tracker. Or you can get its fork, Aniyomi which has anime support and sources as well, not just manga.
- Been using tachiyomi for years and its been great, maybe I should give a try to aniyomi

Phonograph plus if you are a local music nut like me and obtainium if you prefer to not use fdroid to manage your foss app updates
- Is it better than Vinyl?
  
  Imo yes since iirc its the only one besides vlc that actually read the metadata of my music files correctly

MiX
Very good file explorer and server
- I have used MiXplorer since 2016, it's really good. Although I wish it got the new Material Design UI (but that doesn't stop it from being a good application).
  For the people who want to try it out first or don't want to get it from the play store, here's the link to it on Apkmirror.

Minimalist phone. If you have ADHD and/or want to increase productivity or get anxiety about your phone notifications, look into this app. It's been sooooo beneficial for me.
- In this vein I also recommend the KISS launcher on F-Droid. It's super clean. Highly recommend if you like focused phone usage.

Just Run Zero to 5K.
For me it's the Golden Age of apps. Easy to use, minimal, good UI, no ads with the ability to buy premium, all the features that you need and non that you don't.
If you want to start jogging I highly recommend it. I think it's perfect.
Edit: I went from barely able to jog an exhausting half mile to jogging 4.5 miles in 13 weeks. It wasn't easy and I think I "failed" a jog twice but having a clear goal with the encouragement of seeing a timer motivate you really helped me.
- Yeah - I followed the Couch to 5K program a couple of times. It's a really good program to build you up to a 5K run.

JINA App Drawer.
My most used app on my device.
Basically lets you swipe up from the home button to reveal a customized grid of apps + app search.
Makes it so:
I never have to go back to my home screen to launch a new app
I can use my entire home screen for widgets and to just look nice
- Similar but, IMO better option, would be Sesame Search & Shortcuts for those who are interested in these types of things.

Into the breach. One of the best mobile games there is. Sadly you can't get it legally anymore without Netflix subscription.
- You couldn't get it at all before Netflix paid to port it to Android could you?

Oto Music (Play Store link) has been my mp3/offline music player of choice for years now. Stable, pretty, performant, and has tag editing features built in. Still gets updates at least once or twice a year, which is all ya really need for an offline player.
Megalodon (Play Store link) is my Mastodon client of choice, tons of little ease-of-use improvements over the official client. Some people might already be familiar with this if they're on Lemmy, but maybe someone looking for a better Masto client will get something from this.
- Oto music is really good, but not open source unfortunately. Replaced it with Retro music player and to be honest it's just as good
  
  Ah yeah it would be nice if it were open source. Retro is pretty good, I remember having stability issues with it when I used it a couple years ago, but if those got ironed out that's awesome.

Buzzkill for control over notifications.
Musicolet is the best audio player I've found.

ResilioSync for synching files between devices (all platforms as far as I know)

Thumb-key and Auxio (music player)

Leon, the URL cleaner. Use it to remove tracking links either before opening them or before sharing them.
https://github.com/svenjacobs/leon

Shattered Pixel Dungeon - a pretty good FOSS roguelike, gets updated every month or so. Can be a bit hard to learn and beat the game for the first time but trying to go farther and farther each run is really fun.
- I got frustrated by the difficulty curve. It's easy for a few levels, and then suddenly impossible. There are enough easy levels at the beginning that it quickly becomes tedious to grind through back to something interesting.
  That said, I'm not a huge fan of the monotonous nature of rogue-like games, and Shattered Pixel is well-done and good-looking. I just wish the level difficulty scaling were more linear.

Been really impressed with the open-source spending tracking app Cashew
- If you're ever on iOS, check out MoneyStats. Its the best budgeting + balance forecasting app I've ever come across :)
  
  I run Android + iPad so I try and exclusively use cross-platform apps haha

Voice - Audio book player Minimalistic audio book player that supports folders with ".nomedia". Great if you want to keep your audio books and music library separated.
- On the subject of audio(books), check out SpeechCentral. Turns anything into same ;)

SpeechCentral, reads almost any text aloud and basically turns anything into an audiobook that follows along (highlights) so you can read and listen at the same time or export audio for hands/eye free listening!

Read You, probably the best RSS reader available on Android. The bad: still in beta and no sync with Feedly (yet).
Sync for Lemmy, also the best client for Lemmy in Android by far. Design and user experience are delightful. Bad: ads with an expensive subscription. Good: with adguard DNS ads disappear.
- What do you think about Voyager?
  
  Great app but maybe too similar to the rest of Lemmy apps.

Feeder an RSS feed reader. It is amazing, reading all the news I care about without leaving Feeder. It saves me a lot of energy declining cookies. It is available on F-Droid.

I like Weather Warbler because it's simple. Not perfect, but free, simple, no ads.

YouTube Revanced Extended - No ads, almost endless customisation options, built in Return Dislikes and Sponsorblock
SmartTube Next - Same thing for Android TV devices
Boost for Lemmy - The best Lemmy client as far as I'm concerned, highly customisable too
AIMP - No nonsense music player with a clear UI

OruxMaps is an awesome alternative to Google maps. It does require some configuration to get Google imagery working since they were required to remove it, but there's also a huge variety of other online sources it can use (wmts etc), plus off-line maps, overlays. You can use your own maps from qgis or other gis software, and there's multiple navigation options. Tracks, routes, way points, and so much more.
- MagicEarth's amazing too

AnyList. I've tried many list apps and this one is easily the best. Not FOSS disclaimer. The free version has very minimal ads, it has a great, intuitive UI, you can create custom list types and item categories, it has built in quantity and unit of measure fields, and you can share/sync a list with others by email address. The amount of features included in the free version is uncommon in the market without getting more harassed by advertisements. I hope it doesn't change in the future; it's remained relatively unchanged for the 3 years I've been using it!

HiPER Scientific Calculator:
A good calculator with unit conversion (there is a free version with ads and a pro version).
Total Commander:
Good file-manager in connection with plugins for LAN, WebDAV and FTP
WireGuard:
Easy VPN Client
VLC media player:
Nice gestures for brightness/volume/skipping. Many supported file formats.
Perfect Viewer:
CBR/CBZ viewer with customizable gestures
Xodo:
PDF viewer
- +1 for the WireGuard app.