Nagios has, for a long time, been the defacto standard tool for server monitoring. These day that’s surprising, as Nagios’ visible development has been quite slow over the past years and new solutions have popped up meanwhile. Not just self-hosted solutions, but cloud-based solutions with fancy metrics voodoo as well.
Still, though, switching monitoring solutions can be a pain and cloud-based solutions arguably cost more than a self-hosted solution. So, say you want to switch from Nagios or you want to start with server monitoring, there is a solution. Icinga2!