昔はAquaだけだったが、10.3以降は、Mac OS X lookをもつWindow managerと
AppleのX実装がOS Xに含まれいるはず。。

Since version 10.3, Mac OS X has included X11.app, the company's version
of the X11 graphical interface for Unix applications, as an optional
component during installation. Apple's implementation is based on XFree86 4.3
and X11R6.6, with a window manager which mimics the Mac OS X look,
closer integration with Mac OS X, and extensions to use the native Quartz
rendering system and to accelerate OpenGL. Earlier versions of Mac OS X can
run X11 applications using XDarwin.