Quote: The F-Droid Repository is an easily-installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The server contains the details of multiple versions of each application, and the Android client makes it easy to browse, install them onto your device, and keep track of updates.

Quote: Fossdroid, a Marketplace for Open Source Apps → Featuring source links, bug trackers and compiled APK downloads, Fossdroid is a well executed manifestation of a directory of open source Android apps, in a clean and Google Play-esque interface inspired by Material Design, aimed at satisfying your source code requests across a wide category of apps.

Quote: [REPO] The library thread → Here is a list of several libraries put together, so you can use with your own apps for additional features.