The X window server for Darwin and Mac OS X provided by the XFree86 Project, Inc. is called XDarwin. XDarwin can run in three different modes. On Mac OS X, XDarwin runs in parallel with Aqua in full screen or rootless modes. These modes are called Quartz modes, named after the Quartz 2D compositing engine used by Aqua. XDarwin can also be run from the Darwin console in IOKit mode.
In full screen Quartz mode, when the X Window System is active, it takes over the entire screen. You can switch back to the Mac OS X desktop by holding down Command-Option-A. This key combination can be changed in the user preferences. From the Mac OS X desktop, click on the XDarwin icon in the Dock to switch back to the X window system. (You can change this behavior in the user preferences so that you must click the XDarwin icon in the floating switch window instead.)
In rootless mode, the X window system and Aqua share your display. The root window of the X11 display is the size of the screen and contains all the other windows. The X11 root window is not displayed in rootless mode as Aqua handles the desktop background.
From the console, the Darwin port of XFree86 uses the IOKit for device driver loading and discovery, rather than the XFree86 module loader. Because of this, the XFree86 configuration file is not used on Darwin or Mac OS X systems. The descriptions found elsewhere of devices supported by XFree86 for other operating systems also do not apply.
XFree86 for Darwin or Mac OS X will work with any video card that
you have an IOKit driver for. If you are running Mac OS X, this
typically means any card that has been shipped by Apple. If a card works
for you with the Mac OS X GUI, it will work on the same machine with
XFree86. If you have a pure Darwin installation, you may need to download
additional third-party drivers from
Apple's Darwin site. IOKit drivers are typically installed in
/System/Library/Extensions/ or /Library/Extensions/.
Multi-button and scroll wheel mouse support works well with all USB mice that I have tested. I have done no testing with ADB mice, but without a special driver, only a single mouse button will work on an ADB mouse.