Icinga

The Icinga 6-in-1 Stack is a comprehensive and enterprise-ready monitoring solution that integrates easily in existing infrastructures and is unbeatable in configuration possibilities, automation and scaling. Across your entire infrastructure, Icinga gives you the power to watch any host and any application and monitor private, public, or hybrid clouds. Icinga is perfect for large and diverse environments. Hundreds of plugins are available in the official library or simply write your own ones. The software is open-source and free but official support is available via annual subscriptions.