OpenNMS Horizon is described as 'OpenNMS is an open-source network monitoring platform that helps you visualize and monitor everything on your local and distributed networks' and is a Network Monitor in the network & admin category. There are more than 50 alternatives to OpenNMS Horizon for a variety of platforms, including Linux, Windows, Web-based, Self-Hosted and Mac apps. The best OpenNMS Horizon alternative is Zabbix, which is both free and Open Source. Other great apps like OpenNMS Horizon are Nagios, Datadog, Spiceworks and Prometheus.