Pushover is a simple push notification service that integrates easily into web apps like IFTTT, network monitoring systems, security cameras, shell scripts, IoT devices, and anything else that needs to send alerts.
Especially on iOS, it is not easy to implement push notifications for small developers as it costs real money and dev accounts etc. Apps that poll for updates on iOS also get "put to sleep" by the OS.
So Pushover is actually quite a practical solution for small developers as the end user (individuals) pays a once-off cost per platform and then no further usage costs, with Pushover providing the actual push notification service. Devs can push from their app using command line, Python, Ruby, Go, Perl and PHP (examples given for these, but others can be possible too). Each user can receive up to 10,000 messages per month for free.
Pushover itself is not open source, but for a greater variety of push mechanisms via other 3rd party services such as Telegram, Discord, e-mail, IFTTT, Matrix, Pushbullet, Pushover as well, Mattermost, RocketChat, Slack, Teams, and more the free and open source Shoutrrr can be used. Shoutrrr is free and can be self-hosted and most of the services it pushes to are free too.
See Pushover: Simple Notifications for Android, iPhone, iPad, and Desktop
Pushover makes it easy to get real-time notifications on your Android, iPhone, iPad, and Desktop (Android Wear and Apple Watch, too!)