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 two variants of hub in common use:
Stuart Booth's .NET based hub for windows, which you can find here. This comes in 3 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, many xAPplications offer a built-in Hub role if no Hub is detected when they start.
Currently I am only aware of xAPFramework.NET-based applications offering this behaviour.