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.

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.