Zabbix - Track, record, alert and visualize performance and availability of IT resources
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
Arbor - Easily manage product development
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.
Cacti - Cacti is a complete network graphing solution. It provides a fast poller, advanced graph templating, multiple data acquisition methods, and user management features.