Icinga - Monitoring system, A Nagios fork

Icinga is an enterprise grade open source monitoring system which keeps watch over networks and any conceivable network resource, notifies the user of errors and recoveries and generates performance data for reporting. Icinga is a fork of Nagios and is backward compatible. So, Nagios configurations, plugins and addons can all be used with Icinga. Icinga sends notifications via email, SMS, Twitter, instant messaging. It has more features than Nagios. It is distributed and scalable.

Sourceforge location: http://sourceforge.net/projects/icinga/