From Wikipedia: "Xgl is an X server architecture, started by David Reveman, layered on top of OpenGL via glitz. It takes advantage of modern graphics cards via their OpenGL drivers, supporting hardware acceleration of all X, OpenGL and XVideo applications and graphical effects by a compositing window manager such as Compiz. As of May 2006, the Xgl X Server (and related components including the Compiz compositing manager and associated graphical config tools) ships as a non-default in one major Linux distribution, SuSE 10.1, and is included in Frugalware Linux. Xgl can be set up fairly easily for Ubuntu 6.06 (Dapper) because of unofficially distributed binary packages. Xgl is also available as an overlayed package in Gentoo Linux.
Xgl technology requires good OpenGL performance, along with several unique features of recent 3D cards, and presently these can only be accessed using binary-only (proprietary) kernel modules for ATI and Nvidia cards (technically the drivers use a binary-only component coupled to open source code elsewhere). There are some open source drivers for these cards but they allow 2D only, or allow primitive OpenGL 3D capabilities."