Both iperf3 and iperf (v2) is supported

$ iperf3 -c iperf.volia.net -R -P 4

Please visit iperf3 homepage for details.

$ iperf -c iperf.volia.net -r -P 4

See iperf documentation for details.

Or visit its homepage.

Version of at least 2.0.5 is required for bidirectional (upload/download) test to work.

Love GUI? Windows user? Try jPerf.
You'll need Oracle/Sun Java Runtime Environment.