Zabbix - Track, record, alert and visualize performance and availability of IT resources
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.
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.
Ettercap - Ettercap is a suite for man in the middle attacks on LAN.
Prometheus - An open-source systems monitoring and alerting toolkit.
tcpdump - tcpdump is a common packet analyzer that runs under the command line.