What does a Hub do?
As the name suggests, a hub allows one or more xAP applications to share a connection to the xAP network on a single PC. This is a technical requirement because of the way PC networks work - by default, only one application can own a network connection on a given port at a time.
There are three variants of hub in common use:
Stuart Booth's .NET v1.1 based hub for windows using xAP Framework.Net, which you can find here. They come in three flavours: GUI, console and Windows Service.
Other Non Dedicated Alternatives
Whilst operating more than a single xAPplication on a single PC requires a Hub to be operational, some xAP aplications offer a built-in hub role if no hub is detected when they start. Applications developed using xAP Framework xFx have this functionality.