Icinga 2
https://github.com/Icinga/icinga2/

Icinga 2 is a general-purpose monitoring application and the next generation after Icinga 1.x - which was a Nagios fork. It should fit the needs of a small environment as well as big installations.

Rewritten from scratch in C++, with multi-threading and cluster support.

Features:

 * All common features of Icinga and Nagios
 * Much faster and more scalable than Icinga 1 and Nagios
 * New configuration format, more intuitive and template based
 * Monitoring several services by executing checks (see nagios-plugins*)
   for example ICMP/ping, SMTP, POP3, HTTP, NNTP, or other TCP port
 * Any small script can be used as a check plugin, when following the Nagios
   plugin API
 * Notifications about alerts with any custom script, or shipped examples
 * Native support for Livestatus and Graphite