Xgl is an X server architechture which is built upon OpenGL. Doing this allows for lots of very nice eye candy, but it will also require high end graphics hardware. For a demonstration of some of the things possible with Xgl, see the videos in the Links section.
Note that much of the functionality of Xgl has been implemented in AIGLX for the standard X.org X server. This is already enabled by default in Debian Etch and later.
Currently Xgl is in a very early development phase. No release has been made yet. The closest they have come to a release is making a 0.0.1 tag in their CVS repository. Since it is still in rapid development, and no stable releases have been made, It relies on bleeding edge versions of other software such as Mesa and and Glitz. Because of this, it is impossible to package Xgl for inclusion in DebianSid until a stable version is released. It is possible that some version might end up in DebianExperimental soon, but don't hold your breath.
Currently Xgl is only available via cvs:
Here are some external links where you can read more about Xgl:
The Novell page contains some impressive videos that show what will be possible with Xgl:
There are several unofficial repositories providing Xgl packages. Some associated HOWTOs for installing Xgl are:
There is also an Xgl entry on the X Strike Force FAQ.