I have Pushover. I love it. I have a bash alias that I use to notify me when some command completes. I get a notification on my phone, and a tap on my wrist letting me know some hours long task is complete, and I can do the next step.

I have a Octoprint integration which lets me know when 3d prints have completed.

You can also modify the priority of a message, so I can send "critical" events if I need to, although I use that rarely.