ntfy
Simple HTTP notifications to your phone or desktop from any script
ntfy is a simple HTTP-based notification service. A script sends a message with a PUT or POST request, and it appears on your phone or desktop. It suits people who want alerts from their own servers, backups or home devices without signing up for a paid service.
The README describes open-source Android and iOS apps, plus a web app. Messages are published to topics that you subscribe to. The project's documentation covers installing and self-hosting the server, so the notification service can run on your own machine.