What service would you recommend for monitoring systemd services and notify me when one of them are down?

sysvinit

An Arduino with a SIM card.
Sunlight powered.

@c3r38r170 but... why?

I have a couple of systemd services on a server, I just want to know if they are running or not

@catholic-emacs Define pin x as an input and put a signal on it for each systemd service. Message the board and it will tell you if it's recieving or not the systemd services signals.
Make it sunlight powered so in case of a power cut, it could still report.
I don't know why I imagined them as remote, but if they are in a computer you have, you can set up a control panel I guess.
Of course I was kidding about the Arduino.

Can't you build that into systemd itself? Just add SMTP info.

If you can't, you could just write a script that checks service status and emails you on specific failures, or just any failures, and put it in your crontab.

@bahua is that possible? I never knew systemd had such feature