Ask TUAW - Finding which processes own which network connections - Here, I'll Save You 30 Bucks

A good question popped up on Ask TUAW today:

Richard asks
I have recently noticed that I have a fairly constant upload of 14-16 KB/s going on in the background (I'm using iSlayer's iStat menus), even though I haven't initiated any upload. I'm not now running any online backup apps. I've checked in Activity Monitor, and can't see anything very out of the ordinary there, although evidently I've missed something. Is there an easy way to see which applications/processes are using the network connection? Or should I get Little Snitch? It's quite annoying to see that the network connection is being used, and being unsure about what is using it. It's potentially disastrous when I'm using metered connections!

Here's the answer TUAW gave:

Little Snitch ($29.95) is probably the easiest way to go. There's also another recently released net monitoring application called ProteMac Meter ($29.95) which might be worth checking out. I don't recommend Glowworm FW as I've had serious problems with it under Leopard and have found others reporting similar difficulties. It may be that the demos of one of these application will be enough to find the source of your unknown network traffic.

Ummm, let me save you 30 bucks and a download, dude, OS X has everything you need built in:

lsof -i

Lists open sockets. Output looks like this, presto!

COMMAND     PID     USER   FD   TYPE    DEVICE SIZE/OFF NODE NAME
Quicksilv   784 jpdaigle  253u  IPv4 0x4baba68      0t0  TCP 192.168.1.121:64139->host44.hrwebservices.net:http (CLOSE_WAIT)
SystemUIS 13716 jpdaigle   10u  IPv4 0x431a880      0t0  UDP *:*
Mail      23859 jpdaigle   17u  IPv4 0x51cb270      0t0  TCP 192.168.1.31:55041->webaccess.hostedmail.net:imaps (ESTABLISHED)
Mail      23859 jpdaigle   18u  IPv4 0x6d7d270      0t0  TCP 192.168.1.31:55043->webaccess.hostedmail.net:imaps (ESTABLISHED)
Mail      23859 jpdaigle   23u  IPv4 0x8587e64      0t0  TCP 192.168.1.31:55049->webaccess.hostedmail.net:imaps (ESTABLISHED)
Mail      23859 jpdaigle   26u  IPv4 0x58f8e64      0t0  TCP 192.168.1.31:55044->webaccess.hostedmail.net:imaps (ESTABLISHED)
firefox-b 25763 jpdaigle   31u  IPv4 0x703266c      0t0  TCP 192.168.1.31:55088->64.26.141.78:http (ESTABLISHED)
firefox-b 25763 jpdaigle   36u  IPv4 0x9779270      0t0  TCP 192.168.1.31:55089->64.26.141.71:http (ESTABLISHED)