Icinga is a free, enterprise monitoring system that lets you watch the uptime and performance of your important IT infrastructure like computer systems, networks, storage devices etc. It is an enterprise ready monitoring system, provides almost similar look and feel as Nagios and is highly extensible. You can tailor it to your needs and it helps you keep your IT infrastructure up and running. It can monitor unlimited number of systems and notifies you with alerts as soon as any critical system or service is down.