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

Solution 1:

If you don't want to buy any fancy GUI software, you can try nettop in Terminal.app

Use the arrow keys or w or s keys for scrolling. Use -n option to disable IP address reverse resolution.

Example of an output:

                                                               interface           state      packets in        bytes in
iTunes.35506                                                                                           0           0 B
    tcp6 *.3689<->*.*                                                             Listen
    tcp4 *:3689<->*:*                                                             Listen
    tcp4 *:57929<->*:*                                                            Listen
Last.fm.35511                                                                                          0           0 B
    tcp4 127.0.0.1:33367<->*:*                                       lo0          Listen
    tcp4 127.0.0.1:32213<->*:*                                       lo0          Listen
Spotify.35589                                                                                        380         227 KiB
    tcp4 192.168.2.18:57621<->192.168.2.15:52137                     en1     Established             194          34 KiB
    tcp4 *:57621<->*:*                                                            Listen
    tcp4 *:49858<->*:*                                                            Listen
    tcp4 192.168.2.18:58339<->193.182.8.12:4070                      en1     Established             186         192 KiB
    tcp4 127.0.0.1:4371<->*:*                                        lo0          Listen
    tcp4 127.0.0.1:4381<->*:*                                        lo0          Listen

Pretty awesome right? Best of all, nettop is embedded in OSX (well at least in Mountain Lion).

Solution 2:

Rubbernet looks good.

Rubbernet provides a breakdown of per-app network usage, so you can quickly detect apps that phone home, connect to certain servers without your knowledge, or blame the app that's slowing down your network.