Been looking for a tool like iptraf or nethogs for OSX. nettop is ok.
My favorite thus far are these two dtrace scripts for tracing network connections:
soconnect.d: http://dtracebook.com/index.php/Network_Lower_Level_Protocol...
soaccept.d: http://dtracebook.com/index.php/Network_Lower_Level_Protocol...
If you're on OSX and want to play around with dtrace, there are a bunch of really cool built-in scripts:
man -k dtrace