ntfy is a simple HTTP-based pub-sub notification service. It allows you to send notifications to your phone or desktop via scripts from any computer, and/or using a REST API.

curl \
  -d "Backup successful 😀" \
  ntfy.sh/mytopic

Free notifications!

UPD. Used it in my SystemD service files to notify me of service restarts. It was extremely easy to do so. Will use it for now, until I migrate to a proper monitoring system.