I like LibreOffice Draw for this.

I use LibreNMS, which is a fork of Observium. It is primarily SNMP polling, so if you haven’t worked with SNMP before there can be a bit of a learning curve to get it set up. Once you get the basics working it’s pretty easy to add service monitoring, syslog collection, alerting and more. And since it’s SNMP you can monitor network hardware pretty easily as well as servers.

The dashboards aren’t as beautiful as some other options but there is lot to work with.

Monit works for me. Good basic monitoring solution that can also restart a service/interface.

I also use LibreNMS to do alerting for a variety of conditions (syslog events, sensor conditions, outages and services via nagios). But this is more work to get set up.