Prior to the introduction of Mac OS X, virtually the only way developers could create a graphical application in a UNIX-based operating system was with the X Window System, more commonly called X11. X11 for Mac OS X offers UNIX users the ability to run thousands of X11 applications concurrently with other Mac OS X applications. This one goes to 11. X11 for Mac OS X offers a complete X Window System implementation for running X11-based applications on Mac OS X. Based on the de facto-standard for X11, the open source XFree86 project, X11 for Mac OS X is compatible, fast and fully integrated with Mac OS X. It includes the full X11R6.6 technology including a window server, libraries and basic utilities such as xterm.
Mac OS X 10.7