Enterprise Open Source Monitoring with Icinga 1
Icinga is a fork of Nagios developed by an international group of open source monitoring specialists.
Mid-2014 Icinga 2 was released - a complete new development in enterprise montoring.
If you want to migrate from an existing monitoring system or plan to install a completely new monitoring system, then Icinga 2 is the tool of choice.
All current Icinga 1 users can rely on the large community, which will maintain Icinga 1 for many years.