nfsen-ng - nfsen-ng aims to be an in-place replacement for the ageing nfsen.
PagerDuty - Cloud based monitoring service
Zabbix - Track, record, alert and visualize performance and availability of IT resources
VictorOps - We make on-call suck less & help teams to solve problems faster.
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
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.