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.
nfsen-ng - nfsen-ng aims to be an in-place replacement for the ageing nfsen.
AppDynamics - Get real-time insight from your apps using Application Performance Management—how they’re being used, how they’re performing, where they need help.
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
LibreNMS - A fully featured network monitoring system that provides a wealth of features and device support.