ntfy

ntfy is a really cool project to send messages around between devices. It is Open Source, runs on multiple platforms and the service can be self-hosted. - first read about it here - docs - git project - Android App in Google Play Store - Android App F-Droid Store

Sadly this project seems a bit stalled. There are hardly any new commits. Issues aren't handled and the youngest release is from 2018 (last checked January 2023).