What are some alternatives?
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Icinga - Icinga is a fork of Nagios and is backward compatible.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Spiceworks Help Desk - Help desk software features that work for IT
LogicMonitor - LogicMonitor is the SaaS performance monitoring platform for the world's best IT teams. Deploy Fast, Monitor More, Improve Ops.
LibreNMS - A fully featured network monitoring system that provides a wealth of features and device support.