Enterprise grade open source monitoring systemhttp://www.icinga.org/
Icinga is a fork of Nagios and is backward compatible. So, Nagios configurations, plugins and addons can all be used with Icinga. Though Icinga retains all the existing features of its predecessor, it builds on them to add many long awaited patches and features requested by the user community.
Icinga is an enterprise grade open source monitoring system which keeps watch over a network and any conceivable network resource, notifies the user of errors and recoveries, and generates performance data for reporting. Scalable and extensible, Icinga can monitor complex, large environments across dispersed locations. Icinga is licensed under GPL V2 and is free to use, distribute and modify.