iPerf

iPerf is a small, open-source, cli app designed to run on Linux (but has been ported to Windows) which is very useful in measuring throughput on a network.

It runs between two machines on a network, and has various options and features viewable on the man pages (or --help for Windows).

One machine acts as the server;
iperf -s
And the other machine acts as the client;
iperf -c [server ip]

Some useful options :

-t x : This sets the time (in seconds) to run the test for, default is 10 sec
-P x : The number of parallel transfers to run, this will generally increase throughput
-w xKB : TCP window size, increasing this will generally increase throughput as well

Example:

iperf -c 192.168.0.2 -t 30 -P 10 -w 256KB