I've written a WCF Service hosted by a Windows Service and it needs to listen on a known TCP/IP port. From what range can I safely allocate a port for use within my organization? That port will be embedded in the config files for the service and the clients that are consuming the service.
Pick a port number from 49152 through 65535.
IANA publishes a list of currently assigned ports.
http://www.iana.org/assignments/port-numbers
The Dynamic and/or Private Ports are those from 49152 through 65535. This is the range from where you SHOULD pick a port for your in-house applications. Of course any port belonging to one of the unassigned ranges on the published list can be used. But be aware that by picking a port number from those unassigned ranges there is no guarantee whatsoever that the port you choose will not be a reserved port in the future.
UNASSIGNED PORT NUMBERS SHOULD NOT BE USED. THE IANA WILL ASSIGN THE NUMBER FOR THE PORT AFTER YOUR APPLICATION HAS BEEN APPROVED.
And make sure that the port number you pick is configurable as you stated:
That port will be embedded in the config files for the service and the clients that are consuming the service.
This will avoid headaches in case some other 3rd party you-cannot-touch software is using your port number. If that happens you just go ahead and change it on the configuration file and it just works.