Spectrum Spectogram Analyzer was designed with accuracy and easy of use in mind. It can be used for monitoring or measuring environmental sound or noise; help tuning instruments; timing short sound events; and educational purpose.
Analyze and monitor your network with this open source tool.
Worth mentioning : 
Net Monitor (PFA)
“Data Monitor" is a user-friendly, free and open source app for you to manage your data usage. “Data Monitor" helps you to accurately measure your daily data traffic, and analyze the data in a way easy to understand.
A simple dialer app from Simple Mobile Tools
Remember that SMS is not a secure way to communicate !
The king of SMS apps (even better than stock non FOSS ones)
Worth mentioning : 
Simple SMS Messenger
THERE IS NO OFFICIAL F-DROID REPO FOR THIS APP!! You can download the de-googled version from the CalyxOS repo
The best simple to use and complete encrypted messaging app for everyone (except those who wants anonymity !
Worth mentioning : 
Molly An independent fork of 
Signal which let you communicate with Signal users that adds extra-security features. Comes in two flavors like Signal one depending on GoogleMobileServices for notification and one who is independent.
Completely different FOSS messaging apps from the official f-droid repo but also worth mentioning :
aTox
Status
Wire Collect some informations and some metadata aren't encrypted !
Telegram FOSS ( CONVERSATIONS AREN'T ENCRYTED BY DEFAULT!!!)
NOT ON THE OFFICIAL REPO ! Threema official F-Droid repo Threema as now a completely FOSS client on f-droid called Threema Libre ! It's a paid (one time payement) service but you don't need to use a phone number to use it, even if it asks for it just skip it. Everything is E2EE and all the servers are located in Switzerland were privacy laws are stronger than the rest of the world. It collect a really small amount of information to make the service function.
NOT ON THE OFFICIAL REPO ! Session official F-Droid repo
Started as a decentralised Signal fork, now it's an independent project.
It is more private and does not required your phone number so it can be used anonymously ! It also anonymised the traffic !
It is (yet) not as polished as Signal ! Video and Voice calls are in beta !
CAN'T BE USED WITH Signal USERS !
Element is a Matrix-based end-to-end encrypted (E2EE) secure collaboration and messaging app. Developed by the ones behind the Matrix protocol. It provides instant messaging, voice and video calls, screen sharing and file sharing; for 1:1s to huge chat rooms. It’s available to use across Web, Android, iOS, macOS, Windows & Linux. Its decentralised design delivers digital sovereignty, enabling deployment on-premise or through any cloud provider.
Worth mentioning : 
Syphon
A true P2P phone application for voice and video calls. No Internet access needed, no registration, not advertisment or calling back "home". Scan each others QR-Code and call each other. This works in over local networks such as community and company networks or at...
If you really want to use facebook you should delete the app and use one of theses :
Manyverse is a social network app using the SSB protocol (Secure Scuttlebutt) where you can write posts and share with friends nearby or over the internet. It's different from mainstream social networks because your data is yours, it lives on your phone, not in the cloud.
THIS APP IS DISCONTINUED DUE TO META (FACEBOOK) RESTRICTED POLICY FOR THEIR DATA COLLECTING MONOPOLY !
Having the PWA (Progressive Web Apps) in a sandboxed browser or 
Native Alpha could be a better alternative. If you want to use it without any account check Bibliogram Do your own researches !
If you just want to see some Instagram content but don't want a Meta (aka Facebook) app to spy on your phone just use barinsta. It's customizable and is more oriented in a detox use of instagram instead of endless scrolling and double-tapping sessions. You can use it without logging in but sometimes Facebook will temporarily bans your IP so sadly it's recommended to be logged in...
PixelDroid is a Free and Open Source Android client for 
Pixelfed , the federated image sharing platform.
Browse feeds and profiles, upload new posts, discover posts, interact with others on the fediverse.
The OG FOSS Reddit client Feature rich and privacy friendly
Worth mentioning : 
RedReader
My personnal favorite reddit client. Beautiful UI, privacy-friendly, No-ads and quite feature rich
Worth mentioning :
Dawn for Reddit
Stealth The most beautiful client IMO. The app doesn't even let you log in with a Reddit account.
A mobile client for 
Lemmy the open source federated 
Reddit alternative.
Worth mentioning : 
Jerboa for Lemmy
A free, open-source 
X and Nitter client for Android with better privacy. Accountless !
A fast client for twitter without tracking (trakers), you can still connect to your account. Almost all functions of the twitter app.
NOT ON THE OFFICIAL REPO ! IzzyOnDroid repo
The official mastodon client. A federated alternative to Twitter !
Worth mentioning :
Tusky The most
Husky based on Tusky
Fedilab Also a client for Mastodon and other federated social network
It replaces the handwritten diary and helps the user to quickly and easily record, evaluate and export his blood sugar and other important data as PDF or CSV. Thanks to the clearly arranged interface, the user always has an overview of his diabetes. The app also provides information on several thousand foods including carbohydrates and other nutrients.
Sobriety is simple and minimal app to track how long you've been sober from anything you want.
Can I Drive allows you to track your recent alcohol consumption, to assess if you can drive or not.
The Privacy Friendly Pedometer stores the user's step count per hour. The user can review the recorded steps in a daily, weekly and monthly report. These reports also display the calculated distance and calories. As a bonus feature the app can record training sessions (e.g.
Feeel is an open-source Android app for doing simple at-home exercises. It holds the acclaimed full body scientific 7-minute workout regime and allows creating custom workouts as well. While the app currently contains a limited amount of exercises, the plan is to drastically expand the number of both exercises and workouts with the help of the community.
Worth mentioning : 
openWorkout
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