Nagios is a free opensource enterprise-class monitoring system released under GPL License. It allows you to gain insight into your network and fix problems before customers know they even exist. It’s stable, scalable, supported, and extensible. Nagios is Stable, Reliable, and Respected Platform with 10 years in development scaling to 100,000+ nodes. From my personal view, Nagios is one of the best if not the best monitoring system and being opensource makes it that extra special. Nagios is simple and at the same time very flexible made possible by the plugin architecture and most importantly as the author puts it, it just works.