Cockpit. Total gamechanger for me. I don’t have to ssh to my boxes individually to see what’s going on. Instead it’s a web UI AND you can connect basically every server you want to one instance.

https://github.com/cockpit-project/cockpit

That said. For individual servers?

htop/nmon (Get nmon straight from the source! https://nmon.sourceforge.io/pmwiki.php) for a birds-eye sysmon view of things

ps -ef | grep <whatever you're looking for> for investigating stuck/hung/zombie processes

dmseg

tail -f /var/log (or whatever log file you care about)

netstat for net statistics (i think ntop used to be a thing but idk anymore>…)

df -h to look at disk usage

those are all that come to mind right away.

EDIT: formatting and quick explanation of what each tool does