Alignak!

An uncompromising mature enterprise-level platform designed for real-time monitoring of millions of metrics collected from tens of thousands of servers, virtual machines and network devices, effortlessly scaling to even larger environments.

Icinga is a fork of Nagios and is backward compatible. So, Nagios configurations, plugins and addons can all be used with Icinga. Though Icinga retains all the existing features of its predecessor, it builds on them to add many long awaited patches and features requested by the user community. Icinga is an enterprise grade open source monitoring system which keeps watch over a network and any conceivable network resource, notifies the user of errors and recoveries, and generates performance data for reporting. Scalable and extensible, Icinga can monitor complex, large environments across dispersed locations. Icinga is licensed under GPL V2. »

Shinken is an open source computer system and network monitoring software application compatible with Nagios. It's written in Python and therefore is able to run on all platforms where a Python interpreter is available.In contrast to Nagios it is not a monolithic system. Shinken runs as a bunch of highly specialized processes, avoiding the bottlenecks of a single-process design. This parallelity allows to run Shinken not only on one single server. It's components can be distributed over a whole server farm, which results in incredible scalability.Shinken is actively developed and already got more features than Nagios (like priorities for objects, business processes, auto-generation of services, discovery,...) »

Centreon is a GUI built ontop of Nagios.

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