LineageOS is a free and open-source operating system for smartphones and tablets, based on the official releases of Android by Google. It is the continuation of the CyanogenMod project.
CopperheadOS is a hardened mobile open-source operating system by Copperhead Security and based on Android. It aims to provide stronger security and privacy. It also contains a hardened kernel and sandbox features for app isolation. Available for select Pixel and Nexus devices.
Sailfish OS is a mobile operating system combining the Linux kernel for a particular hardware platform use, the open-source Mer core middleware, a proprietary UI contributed by Jolla, and other third-party components.