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
A powerful open source app to keep track of your body. Supports users, no-cloud sync so you keep your data for you and many other like :
Support various Bluetooth scales Logs your body metrics (Weight, BMI, waist-hips raito and more) Keep track of your diet process Display all your data on a chart and table Import or export your data from/into a CSV file and more...
NOT ON THE OFFICIAL REPO ! IzzyOnDroid repo
A beautiful simple open source Body Mass Index Calculator
Which vegetables and fruits are from the current season ? Eat better with this calendar. (Central Europe only for now)
Open Food Facts is a database of food products with ingredients, allergens, nutrition facts and all the tidbits of information we can find on product labels.
Made by everyone - Crowdsourcing
1800+ contributors like you have added 75 000+ products from 150 countries
Also check : 
Open Pet Food Facts
Open Beauty Facts
Open Products Facts
An app that help you getting healthier following recommandation that Dr Greger write in his book How Not to Die and his website nutritionfacts.org
Grocy Android is an open-source Android client for grocy. Grocy is a self-hosted groceries & household management solution for your home.
A menstrual cycle tracking app that's open-source and leaves your data on your phone. Use it to track your menstrual cycle and/or for fertility awareness! Not a pink "girly" app ^^
If you know how to package for F-Droid and have time join their GitLab !
Worth mentioning : 
log28 (official f-droid repo)
Brethap is yet another meditation app (yama). Including a timer for meditation, it also allows you to configure a breathing pattern for you to follow. Your sessions will be saved, allowing you to monitor and view stats about your progress in either a list or calendar view. Duration, breath and other options are configurable through saved preferences.
Worth mentioning :
Meditation
Meditation Assistant
Om (meditation)
A really minimalist ad-free open source launcher.
Worth mentioning :
Olauncher
Lunar Launcher
NOT ON THE F-DROID REPO ! MM20 Apps repo
A beautiful launcher that is NOT based on AOSP launcher
This is not your typical pixel launcher. posidon launcher was built from scratch, to be a different experience.
Minimal: There are constant improvements and optimizations in the ux, to make all of the settings as easy to understand as possible.
NOT ON THE OFFICIAL REPO ! IzzyOnDroid repo
A really nice open source launcher, quite powerful, ad-free with componements from the popular 
Lawnchair and 
Google Pixel Launcher !
Worth mentioning :
OpenLauncher
Lawnchair (not updated since 2019)
Emerald Launcher
Rootless Pixel Launcher (not updated since 2019)
The default launcher of the de-googled /e/ OS. It is more iOSish than stock Android with no App Drawer, all apps in the main menu and widget on the left panel.
I personnaly don't really like it but hey you might like it !
Muzei is a live wallpaper that gently refreshes your home screen each day with famous works of art. It also recedes into the background, blurring and dimming artwork to keep your icons and widgets in the spotlight. Simply double touch the wallpaper or open the Muzei app to enjoy and explore the artwork in its full glory.
Plugins :
Muzei Ghibli
National Geographic for Muzei
Wikimedia Commons for Muzei
Pixiv for Muzei 3
Doodle is an open-source Android app that provides colorful live wallpapers with auto dark mode and power-efficient animations. The wallpapers are based on the original Doodle live wallpaper collection of the Google Pixel 4. Additionally there is an exclusive "Material You"-inspired live wallpaper included. The app is not just a copy of the original wallpapers, it's a complete rewrite without the permanent animations to save battery and storage space. In addition, there are many customization options to match your preferences.
Worth mentioning:
Dark Mode Live Wallpaper
This is the android port of 
2048 . It's nothing fancy, just a webview that loads the locally stored html files of the original game. It's playable without internet (the app doesn't require any permissions) and...
This is a free (GPL-3.0) open source copy of some popular games full of ads, but without them. Its made with godot engine and here is the source code: https://github.com/Dariasteam/TowerJumper.
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