Icinga is a secure, open-source network monitoring solution. The multithreaded design means it's fast and capable, while offering extreme flexibility. Makes good use of metadata information to give you a better understanding of the state of your hosts and services. Performance data can be sent directly to third-party tools, and it natively supports Graphite and InfluxDB. Setup is undeniably complex, but there is good documentation that can help motivated users get the most from this powerful tool.