StreamWatcher is a network monitoring tool for the Mac. It allows you to monitor in real-time all the current connections in your local network. It can detect both connections originating from your computer and from other devices on the network. You can monitor any interface on your Mac, including the Airport card and your ethernet card.
When you start the application, it will ask you which interface you want to monitor. It lists all the interface which have an IP address, so those that are not connected or active won't be listed. After you select the interface, the active connections will start appearing on the main window. They will only remain on screen while they are active. Each connection shows the protocol (TCP or UDP), the source address, destination address, upload speed, and download speed.
In my testing, StreamWatcher started detecting connections almost instantly. It detected connections that originated from my Mac, and from my iPhone, which was also connected to my home network. I was able to save the log to a file with no problems.