xpra (remote X11 desktop proxy)

Xpra is known as "screen for X". Its seamless mode allows you to
run X11 programs, usually on a remote host, direct their display
to your local machine, and then to disconnect from these programs
and reconnect from the same or another machine(s), without losing
any state. This effectively gives you remote access to individual
graphical applications. It can also be used to access existing
desktop sessions and start remote desktop sessions.

Note: If 'xpra attach' freezes up with "UI thread is now blocked" on
its stdout, try running it again with --opengl=no.