iperf - A TCP, UDP, and SCTP network bandwidth measurement tool
Zabbix - Track, record, alert and visualize performance and availability of IT resources
JPerf - This project gives a better UI and new functionalities to the initial JPerf 1.
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.
NetStress - NetStress is a user-friendly benchmarking tool that is mostly used by networking professionals to measure the network performance of both wired and wireless connections.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services