check_mk is what I use at home and at work, it’s a fork of nagios/icinga, works with agents, nagios plugins, or snmp, and if somehow you can’t find what you want to monitor, writing custom checks is as easy as writing a bash script

I opted for checkmk as well and don’t want to switch. It’s got a good default for Linux monitoring and it will tell me about random things to fix after reboots, or that memory/disc is getting low so I can fix it quickly.

When monitoring 15 virtual machines on one physical the default of checking every minute for all machines raised the temp over 80 degrees Celsius on the physical machine and triggered a warning. Checking every five minutes is more that I need, so I went with that change.