I'm using Pushover (https://pushover.net/) to send notifications to my Apple devices. To make use of it easier on command line I wrote a simple Python script which I've found to be very useful for things like allowing Transmission to send notifications when a download is finished or in scripts that do backup jobs which take a long time, letting me know when my Mac has booted, etc... Reply to this comment if you're interested in my script (note that the pushover site has good examples of how to call their API).