Cacti is an independent network graphing solution which features a fast poller, advanced graph templating, multiple data acquisition methods, easy user management and plugin support. All of this is wrapped in an intuitive, easy to use web interface that scales from small installations up to complex networks with hundreds of devices. Cacti offers out of the box support for most Windows, Linux and Cisco devices over SNMP. It has a thriving community of developers providing extra graph templates, data acquisition methods and plugins.

Icinga is an Open Source network monitoring system, originally forked from Nagios to speed up feature development and add a modern web user interface. It is compatible with Nagios plugins, configurations and addons.

Icinga 2 is an advanced monitoring application with a wealth of features, including native clustering and high availability, a REST API and web-based configuration management.

LibreNMS is a network graphing and alerting system featuring distributed polling, autodiscovery, integrations with a range of complementary tools and support for Nagios plugins. Ideal for MSP and ISP NOC environments.

Naemon is an Open Source network monitoring system based on Nagios Core 4, but with many bug fixes, new features, and performance enhancements. It is compatible with Nagios plugins, configurations, addons and forms the basis of commercial monitoring products.

Nagios is the most well-known and widely used Open Source network monitoring system. With a history going back 20 years, a reputation as a must-have and an easy to understand model, it has an ecosystem of tools, add-ons and monitoring plugins that is unmatched by any other monitoring system, Open Source or proprietary.