Mac command line tip of the day:
...and then press c and d.
This will show you which programs (or more to the point, processes) use your network connection and how much. c collapsed the rows so you don't get one line per connection (you can get back to the expanded view by pressing e). “Delta mode” d would show you how much network capacity each program (or connection, if you're in expanded view) is using right now and pressing d again will take you to showing the total amount of traffic transported.
If you want to get geeky, you can toggle p to see the number of bytes transported rather than the more human readable so and so many megs or gigs.
Use the arrow keys to scroll to the sides for more statistics (use j to select which stats to display) and up-and-down if you have a really large number of programs on the list, or are watching the expanded view.
Finally, h will bring you the help screen so you don't have to remember all the keys i just wrote about :D