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.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
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.
SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services