KDrive (formerly known as TinyX) is a light-weight X server targetting specific
chipsets. It is recommended to be used on thin-clients and embedded systems.
If you are on a standard desktop system you might want to use x11-server-xorg
and the video driver corresponding to your video card.

Xephyr is a 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.

Possible uses include:
- Xnest replacement - Window manager, Composite 'gadget', etc development tool.
- Toolkit debugging - rendundant toolkit paints can be observered easily via
the debugging mode.
- X Server internals development - develop without the need for an extra
machine