Wikipedia
Iftop
iftop is a command-line system monitor tool that produces a frequently-updated list of network connections. By default, the connections are ordered by bandwidth usage, with only the "top" bandwidth consumers shown.
The iftop website gives the following description: "iftop 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?'".