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. This comes in three flavours: GUI, console and Windows Service.
Development of xAP Framework (xFx) its hubs has now been taken over by Edward Pearson. His enhanced xAP Framework v2 hubs are available from here.
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 xFx_v2 have this functionality.