Paul Warren's iftop tool provides quick-n-easy way of seeing the top bandwidth hogging connections on a specified network interface. Just like other top like tools, it's a text console based tool.
To install, use:
sudo apt-get install iftop
To monitor traffic on my wifi interface wlan1, I used:
sudo iftop -i wlan1
..note that specifying more than one interface causes it to segfault :-(
It has a few interactive commands, press 'h' to get help, and 'h' again to toggle back out of the help screen. For example, the 't' key allows one to cycle through send/receive display modes, and the 'p' key toggles on/off the port number on the connection display. Naturally, the 'q' key quits the application.
The bottom of the screen displays a summary of the TX/RX statistics including cumulative totals and peak transfer rates. All in all quite a helpful little tool.