What Are Some Great Android Open Source Alternative Apps?

I've recently decided to switch out my Playstore apps for Open Source Apps as they're usually Ad Free and much less bloated. Can we use this thread to help people find open source android apps?

I'll start

K-9 mail, a great email alternative that let's you have multiple users logged in

Red Moon - A customisable night light app which lets you adjust the colour and settings including Colour, opacity and Darkness

Swift Notes - Note taking app