PushBits is a relay server for push notifications. It enables your services to send notifications via a simple web API, and delivers them to you through various messaging services. For now, only the Matrix protocol is supported, but support for different services like Telegram could be added in the future. I am myself experimenting with Matrix currently because I like the idea of a federated, synchronized but still end-to-end encrypted protocol.

The idea for this software and most parts of the initial source are heavily inspired by Gotify.

Written in Golang.

Source code for the Gotify Android client application. Can also be installed from the Play or F-Droid stores.

A self hosted push notification server with its own REST API. Written as an alternative to Pushover, et al. Written in Golang. API can send and receive messages, manage users, and manage clients and applications. Docs built in at /docs. Has its own control panel, too. Has a mobile app in both the Play and F-Droid stores. HTTPS enabled, can communicate with Let's Encrypt for its certs.

Pushover is a pushnotification site that just gives you an API to work with and a near-universal app that runs on Android or iOS.  Add realtime notification to whatever you want with nearly no work.  The app's free for a week; after that you have to pay $5us for a perpetual license.  You also get 7500 notifications per month per integration.