Xmove
xmove is a piece of software which acts as a virtual X display. This display can be dynamically re-directed to change which actual X server it is displaying things on. So the theory is that you could start an X application, and then later move it so it was being displayed on a different system.
This appears to be ancient (1995 or so) Sun software. A version is in debian but it is crinkly to say the least. There does not appear to be an "official" version anywhere.