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.
Uses your phone's spare computational power to help scientists working on everything from cancer research to finding pulsars and black holes. You choose which projects to contribute to. Phones are more compute-per-watt efficient than most computers, and every little bit helps!
Can't be published in Google Play store due to their ToS (it downloads and executes code from outside of its own APK which is not allowed)
Make sure to limit CPU usage to keep temps down. If your phone can't get rid of heat easily, this app is not for you.
My favorite F-droid app: !boinc@sopuli.xyz
can't trust it (blindly) for some reason... but there are other reasons I will not get into here