CheckMK offers pretty solid monitoring our of the box, but I think it falls quite short of the mark when you want to add more than the default monitoring. It's

CheckMK offers pretty solid monitoring our of the box, but I think it falls quite short of the mark when you want to add more than the default monitoring. It's still probably the easiest solution for monitoring a bunch of basic VMs, servers, etc. You can set it up and get a solid idea of how it works in only a few hours.