What is it? It’s a way to send arbitrary notifications from…well, just about anywhere to just about anything that can “listen”. The author has created an app (iOS and Android) as well as a CLI, and an API that you can run via cURL or any other similar app which means you can drive it just about any way you want!
I came across this in HackerNews at https://news.ycombinator.com/item?id=29715464. The author has the source available on github at https://github.com/binwiederhier/ntfy. While you can host it yourself, the author has the service available for free at ntfy,sh.