ntfy.sh
ntfy (pronounce: notify) is a simple HTTP-based pub-sub notification service. It allows you to send desktop and (soon) phone notifications via scripts, without signup or cost. It's entirely free and open source. You can find the source code on GitHub.
You can subscribe to a topic either in this web UI, or in your own app by subscribing to an SSE/EventSource or JSON feed. Once subscribed, you can publish messages via PUT or POST.
Subscribed topics: