What are some alternatives?
Zabbix - Track, record, alert and visualize performance and availability of IT resources
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.
Sensu - Monitor servers, services, application health, and business KPIs. Get notified about failures before your users do. Collect and analyze custom metrics. Give your business the competitive advantage it deserves.
Icinga - Icinga is a fork of Nagios and is backward compatible.
Munin - PnP networked resource monitoring tool that can help to answer the what just happened to kill our performance
LogicMonitor - LogicMonitor is the SaaS performance monitoring platform for the world's best IT teams. Deploy Fast, Monitor More, Improve Ops.