iperf - A TCP, UDP, and SCTP network bandwidth measurement tool
Multiping - Get MultiPing Support. Network monitoring in 60 seconds. MultiPing provides a graphical view of network performance, monitors hundreds of targets, and sends automatic alerts..
JPerf - This project gives a better UI and new functionalities to the initial JPerf 1.
SmokePing - Smokeping is a tool for measuring latency.
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.
Fping (open source) - fping is a program to send ICMP echo probes to network hosts, similar to ping, but much better performing when pinging multiple hosts.