iftop is described as 'does for network usage what top(1) does for CPU usage. It listens to network traffic on a named interface and displays a table of current bandwidth usage by pairs of hosts. Handy for answering the question "why is our ADSL link so slow?"' and is a Network Monitor in the Network & Admin category. There are six alternatives to iftop for a variety of platforms, including Linux, BSD, Solaris, Xfce and Mac. The best alternative is Nethogs, which is both free and Open Source. Other great apps like iftop are nload (Free, Open Source), ntop (Free, Open Source), Bandwhich (Free, Open Source) and IPtraf-ng (Free, Open Source).
nload is a console application which monitors network traffic and bandwidth usage in real time. It visualizes the in- and outgoing traffic using two graphs and provides additional info like total amount of transfered data and min/max network usage.
NTop is a hybrid layer 2 / layer 3 network monitor, that is by default it uses the layer 2 Media Access Control (MAC) addresses AND the layer 3 tcp/ip addresses. ntop is capable of associating the two, so that ip and non-ip traffic (e.g.
IPTraf is a console-based network statistics utility for Linux. It gathers a variety of figures such as TCP connection packet and byte counts, interface statistics and activity indicators, TCP/UDP traffic breakdowns, and LAN station packet and byte counts.