PortsMonitor is a network monitoring tool for the Mac. It allows you to see what connections each service and application establish on your Mac. The usefulness of this application depends on your knowledge of network systems. If you don't even know what a protocol is, you probably don't need this app. However, for people who at least grasp the concepts of protocols, ports, IP addresses, etc. this can be very useful. One of the many uses for this app is to determine if you have any rogue services or applications connecting to the Internet, and then take action. It should be noted that this application only monitors established connections, it can't block or terminate them.
By default, PortsMonitor auto-refreshes every second, but you can disable this or change the refresh interval. New connections are shown in pop-up windows when the application is auto-refreshing. The main window shows a list of processes, along with their IDs, types, protocols, and ports of each established connection. It also shows the local and remote addresses so that you can know where the connections go.
In my testing, PortsMonitor performed quite well. New connections showed up in the application immediately and all the information seemed accurate.