I use Pushover for a few alerts so years ago I wrote a little bash script called `push` that you pass a title and optionally a body. Was very nice to to `./longRunningCommand && push "Task Done" "Here is a body"`. I'd sometimes combine this with my `beep` script that just makes a noise for when I know I'll still be at my computer but want to know when something finishes.
Here is my beep script, it's almost embarrassingly basic. You can pick a different sound, I wanted one that wasn't too offensive/harsh and I've used this for 3-4+ years.