Run It Yourself

I use NTFY.sh and a few python scripts to let me know about ssh logins, full disks and hosts that stop responding to pings. Very DIY but it works and doesn't rely on outside services.

https://checkmk.com/

I've been slowly setting up and playing with the Raw Edition.

https://healthchecks.io

I use the hosted version because the point of monitoring is that it alerts me when the rest of my stuff is down. You can self host it though.