Ever wonder what's using your internet connection? If you have LittleSnitch you're already there with a nice GUI, but if you're logged on to the system remotely or prefer CLI commands check out 'nettop' in OS X Lion. I noticed this when I was looking for any new network utilities in Lion. This command wasn't there in Snow Leopard or before.
I usually compile and install mtr, ntop, trafshow and other networking tools, but it's not always the safest thing to compile and install executables as root downloaded from the Internet.
Open a Terminal window, and then type nettop. You can also type:
man nettop or nettop --help for more command options.
The nice thing about this vs. other utilities is that it works as a normal user. If you put your Terminal session into fullscreen mode and have a large display (1920x1200 for me) you can see a ton of info about network connections, traffic from to, ipv4 and ipv6, tcp or udp and routing changes.
[crarko adds: As the name of the command implies, this is like top for network activity instead of CPU usage for local processes. I also checked in Snow Leopard and it wasn't included there. I think it's like 'nethogs' for Linux.]
Mac OS X Hints