Here’s a short list and a short description of programs that can be used with Xnee:
Fakes keyboard/mouse input via the kernel (Linux). Used to verify GNU Xnee.
Xephyr is a 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. It uses SHM Images and shadow framebuffer updates to provide good performance. It also has a visual debugging mode for observing screen updates. Xephyr is part of the Xserver module from freedesktop (GIT) Readme
(…. Xephyr is one of Henrik’s absolute favorite tools)
Xvfb is an X server that can run on machines with no display hardware and no physical input devices. It emulates a dumb framebuffer using virtual memory.
Read more at en.wikipedia.org/wiki/Xvfb