"F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device. "  - fdroid

The above sentence pretty much sums it up. Here is why I think FOSS is great:

• It's free of advertisements
• Does not track you
• It's free (althroughtI recommend donating)
• If you are a developer yourself and you really want a feature, you can just contribute it
• The code can be audited by anyone

Browse Apps available on F-Droid here: https://www.f-droid.org/en/packages/