So what's special about this VNC system besides support for only sharing individual windows? Well, it's being developed as a plug-in for GNOME's Mutter so that one can easily share a sole window with their "contact" via using GNOME's Telepathy framework for communication. Both users can interact with the same window simultaneously over the network.
The xzibit system has plans to fully implement Telepathy support (it already works over the loop-back interface, but xzibit 0.0.2 will support the use of Telepathy Tubes), X Input 2.0 support for being able to fake mouse clicks and key-presses on arbitrary windows, and Doppelganger pointers for tracking the movement of pointers on other displays. There is also audio support that's partially underway in a separate branch of this brand new project.
Details on this brand new project can be found in the xzibit release announcement, on the Telepathy Wiki, or in its Collabora Git repository. For now GNOME users have the Vinagre VNC client for connecting to a VNC server within GNOME.