When I want to know what Linux applications are eating the most bandwidth “right now” I like the nethogs utility (emerge nethogs, apt-get install nethogs, etc). It will show you, for a specific or all network interfaces, a list of how much up / down bandwidth each application is eating.
Then, to get nice bandwidth graphics per day or week, if you don’t like or want mrtg and prefer something simpler, then bandwidthd is for you. Bandwidthd is available on Ubuntu (apt-get install bandwidthd).
Done. Two quick tips to get you out of a hurry in bandwidth measurement / bandwidth usage / bandwidth monitoring.