Application for monitoring all applications that are using the internet in Mac OS X

"Try LittleSnitch:

A firewall protects your computer
against unwanted guests from the
Internet. But who protects your
private data from being sent out?
Little Snitch does!"

"If you have some Command Line know how, there are some Unix tools you may be able to take advantage of.

iftop: Won't give you a per application breakdown but it will give you a breakdown by remote host. This means that you can at least see who you're sending information too and how much. It's also installable via DarwinPorts.

nethogs: This one may not run on OS X, though it would be more informative. Unfortunately, it's probably a moot point.

You can also check out this other question for more information: Bandwidth Usage in Linux."

As of version 4, iStat Menus shows the bandwidth usage of the top 5 applications.

"I just tried out Little Snitch, having the same basic problem (it seems) as the OP, and having been so disappointed with rubbernet (I weep for my $40).

Little Snitch is exactly what the OP asks for, and has every feature I can dream of in exploring related problems to those implied by the OP's question.

It does report the total bandwidth used by each application individually, and even presents a graph of any selected application or group of applications! Once you realise that you can select one or more rows in the Network Monitor graph, and ask for the collated info of those by presing the info button, it is simple to use. It survives changes of network status seamlessly.

It has a demo that lasts 3 hours, so you can't go wrong.

I think it is the perfect answer to the OP's question."