Monitor your health without being spied on
Free/Open Source applications also help us in monitoring our health by limiting the control over our health to us and not to commercial companies.
Whether it is heart rate, sedentary lifestyle monitoring, sleep duration, medication reminders, etc., these applications, in addition to having the advantages of proprietary ones, can be useful to improve our quality of life simply and without necessarily sharing such sensitive data on habits and health conditions with companies that could perhaps exploit the data against us, think for example of life insurance companies that could increase or deny policies to obese, sedentary people with a high probability of developing cardiovascular diseases, or pharmaceutical companies that could place their latest product in a certain place due to the number of people suffering from a certain pathology.
So a tool like a simple smartwatch would become our Trojan horse and would instead favor the usual suspects.

Gadgetbridge
The first app that can replace the proprietary smartwatch control apps on our cell phone is Gadgetbridge, compatible with numerous smartwatches, even high-end ones, including Sony, Casio, Fossil, Amazifit, etc., there are currently 319 devices supported by this app, depending on the smartwatch sensors it can manage:
Heartbeat
Distance Traveled
Sleep/Wake Hours
Steps
SMS/Chat Notifications
Call Management
Geolocation
I have tried and use this application and I must say that up to now I have not found any obstacles, the only flaw is that like all open source applications, installation and synchronization are hindered in some cases by the manufacturers for obvious data acquisition reasons.

OpenScale
Are you looking for an app to manage your electronic scale? No?
Then you should do it, yes because in this case too, just like the smartwatch, data on your health status is collected so the options in this case too are 2
Do not use these tools that are useful for monitoring health or use them but be careful that such sensitive data does not end up in the wrong hands.
With OpenScale you can use your smart scale without having to share your data with anyone.
At this link you can find a list of currently compatible scales, many are on sale on Amazon.
To be fair, I must say that the app is not completely free, it is Freeware so not completely free, I have not found better alternatives that read the data directly from the scales.
https://github.com/oliexdev/openScale/wiki/Supported-scales-in-openScale
OpenScale allows you to collect the following metrics:
Weight
Percentage of Fat and Lean Mass
Water present in the body in percentage
Caloric requirement
BMI
A great app for those who are on a diet and want to keep track of various values.

HomeMedicinekit
The application that allows us to control our stocks of medicines, supplements by dividing them into categories, that reminds us of the expiration date, and notifies us when to take the medicine.
Especially for the elderly it is an excellent ally that reminds us when to take the dose and reminds us whether we have taken it or not.

FitoTrack
Want to track your workouts but not share them with anyone?
Fito track allows you, thanks also to the association with gadgetbridge, to track your training sessions, whether it is a simple walk or a bike ride, it then uses open source maps (not those of Google like most trackers) to outline the route you have taken and calculate a series of values such as distance, heart rate alerts, altitude, speed in km/h.
In short, an indispensable companion if you do sports but in a private and above all free way.

Noise: Natural calming noise
It is known that the sounds of nature help to find lost serenity, but it is not always possible with today's pace to find a moment to take a walk in nature in the woods, if you can not get away from the chaos of the city we can at least listen to some relaxing sounds, this application does just that, it allows you to set a series of relaxing sounds from the gurgling of water to the crackling of the fire, to the sound of the wind allowing us to calm our mind at least a little.

MediLog
I think this app can be helpful to elderly people or those suffering from certain pathologies such as diabetes, high blood pressure, asthma but also to keep track of the state of health when we are not in good shape, MediLog allows you to take note of some values such as blood pressure, glucose, weight, temperature, oximetry and collects them in a database perhaps to then submit them to the attention of your trusted doctor.
I think this app is perfect for older people who want to keep track of their measurements, it can also be useful for people in the workplace, such as private nurses, carers for the same reason.
The application also has a notes space for all those people who perhaps want to keep a diary of sensations.
These were just some of the most interesting Open Source applications related to health but there are many others that you can always find on F-Droid or on Alternative to.