Xsgi is the native X11 compatible display server used for most of the IRIX. line (with the exception of the SGI Onyx4, which uses a modified XFree86)
Xsgi is heavily optimized to use IrisGL/OpenGL commands, later standardized via GLX. This is a form of indirect rendering, in contrast to X.Org's DRI extensions. The license of the program is proprietary, with its source never having been released, leaked or even mentioned in SGI literature. The multi-threaded, highly optimized nature of the display server allowed for 2D GL effects on the desktop, including vector icons, graphical special effects, and many common demos.
GLX lives on in X.Org and many other implementations of X. Much like OpenGL and IrisGL, Xsgi's contributions to computer graphics and common desktop effects cannot be overstated.