Many times I find myself behind a firewall or a NAT device and need to quickly determine if a port passing through this configuration is working as expected.  One of the easiest ways, not to mention coolest, is to use the TCP Listener tool created by AllScoop.  This little gem allows you to configure whatever TCP port you want to publish along with a message that can be displayed when a connection is successfully made on that target port.

The one thing I wish it would provide that isn't included is the IP address of the connection.  I have asked for the source code to see if I could make the addition myself but I haven't heard anything back yet.

Here's the screen shot provided by the AllScoop website: