Icinga!

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.

Join millions of IT professionals and download Spiceworks, a free network monitoring software with network management tools, help desk ticketing app, network mapper and more.

Datadog is a monitoring service for IT, Dev & Ops teams who write and run applications at scale, and want to turn the massive amounts of data produced by their apps, tools and services into a unified view of IT infrastructure. Datadog's capabilities are provided on a SaaS-based data analytics platform that enables multiple teams to work collaboratively on infrastructure issues.

Observium is an autodiscovering PHP/MySQL/SNMP based network monitoring which includes support for a wide range of network hardware and operating systems including Cisco, Linux, FreeBSD, Juniper, Brocade, Foundry, HP and many more.

Cabot is a free, open-source, self-hosted infrastructure monitoring platform that provides some of the best features of PagerDuty, Server Density, Pingdom and Nagios without their cost and complexity.

Pandora FMS is an enterprise-ready monitoring solution that provides unparalleled flexibility for IT to address both immediate and unforeseen operational issues, including infrastructure and IT processes. It uniquely enables business and IT to adapt to changing needs through a flexible and rapid approach to IT and business deployment.

Check_MK is comprehensive IT monitoring solution in the tradition of Nagios. Check_MK is available as Raw Edition, which is 100% pure open source, and as Enterprise Edition with a lot of additional features and professional support.

Opsview Atom is ideal for personal use, SoHo setups, test or lab environments or just to get your hands dirty with all of Opsview's monitoring functionality!

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,...) »

Opsview is award winning unified service monitoring software for complex, distributed IT infrastructures.

Alignak is a modern monitoring framework based on Shinken. Its main goal is to give users a flexible and complete solution for their monitoring system.

NetXMS is an enterprise grade multi-platform open source network management and monitoring system. It provides comprehensive event management, performance monitoring, alerting, reporting and graphing for all layers of IT infrastructure — from network devices to business application layer.

The Sensu Open Source Project (a.k.a. "Sensu Core") intends to provide an open framework for building comprehensive monitoring solutions, without imposing restrictions or being overly opinionated. While new user-facing "features" may be developed exclusively in Sensu Enterprise, the framework that makes those features possible will always be a part of Sensu Core.

An open-source monitoring and alerting system