UnifiedPush is a set of specifications and tools that lets the user choose how push notifications* are delivered. All in a free and open source way.

ntfy is a simple HTTP-based pub-sub notification service. It allows you to send desktop notifications via scripts from any computer, entirely without signup or cost. Made with ❤ by Philipp C. Heckel, Apache License 2.0, source at https://heckel.io/ntfy.