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.
Prometheus - An open-source systems monitoring and alerting toolkit.
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
Grafana - Data visualization & Monitoring with support for Graphite, InfluxDB, Prometheus, Elasticsearch and many more databases