iftop is a free network bandwidth analyzer. In the words of it’s creators “iftop is for network usage what top is for CPU usage”. It has many useful features including filtering by protocol and network (f) or an arbitrary regex (l) and is now available as a Universal Binary in installer package format. It’s installed into /usr/local/sbin.
The interface is fairly self-explanatory (the 3 columns on the right are transmission statistics for the last 2, 10 and 40 second intervals) but you might want to check the manpage for a complete explanation. You can also press “h” to get a help screen while it’s running.