X11R7.6™, a freely redistributable open-source implementation of the X Window System from the X.Org Foundation, has been ported to Darwin and Mac OS X.

Most of the current work on X for Darwin and Mac OS X is centered around the XQuartz Project at MacOSforge. If you are interested in up-to-date status, want to report a bug, or are interested in working on X11 for Darwin, stop by the project website at http://xquartz.macosforge.org/