Since we should all try to decentralized our personnal informations and try to find free and open source alternatives of our favorite applications and services, here is a list of website where you will be able search and find what you are looking for.
Feel free to add a pertinent link about alternative apps or services, this post can be edited.
Enter the name of an Android, iOS app or a Windows, Linux, MacOS software or a website name and you will have a list of alternatives for the OS you want, open-source or proprietary, you select your filters.
A list of Google products alternatives.
A list of alternatives for some apps and services, for every platform.
A lot of open-source services hosted by Framasoft. A lot of them can be self-hosted.
A not well organized list of alternatives for a lot of thing, but not really for mobile apps.
A list of free and open source Android apps.
A list of proprietary Android apps and some of their alternatives.