Xpra is 'screen for X': it allows you to run X 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, without losing any state.
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Xephyr AlternativesRemote Desktop Tools and other similar apps like Xephyr
Xephyr is described as 'Kdrive based X Server which targets a window on a host X Server as its framebuffer. Unlike Xnest it supports modern X extensions ( even if host server doesn't ) such as Composite, Damage, randr etc (no GLX support now)' and is a Remote Desktop tool in the os & utilities category. There are 1 alternatives to Xephyr for Mac, Windows and Linux. The best Xephyr alternative is Xpra, which is both free and Open Source.
Xephyr allows a second x11 session to be run within a window locally. Xpra is for remote sessions.