All the talk is about the new eyecandy that will be available in Windows Vista. While Mac users have been enjoying cool features on their desktop for ages, Expose for example, Linux users will soon be able to do the same and maybe more. XGL is a composite layer that runs on top of the regular X-Windows GUI. It allows the desktop to make full use of your powerful 3D graphics card.
Xgl is an X server architecture, started by David Reveman, layered on top of OpenGL via glitz. Nowadays, most PCs are shipped with a 3D graphics card (foremostly from NVIDIA or ATI Technologies) and Xgl allows the X server to take advantage of its 3D processing power, assuming suitable drivers are available.
The results are absolutely staggering. See for yourself: