XFree86
XFree86 is a free and Open Source implementation of the X Window System which runs under Linux, FreeBSD, NetBSD, OpenBSD, Solaris, MacOS X, Windows NT and several other minor flavors of Unix. As of 1 October 2001, XFree86 supported the X11R6.5.1 spec, including the GLX and Render extensions.
The XFree86 project has recently been forked by Keith Packard, creating the XWin project; the new branch of the X server itself has been given the name Xouvert.
Configuration
/etc/X11 includes the configuration files. The basic configuration file is /etc/x11/XF86Config-4 that includes variables about the screen, keyboard and video card. The program xf86config is often used, although xf86cfg also comes with the XFree86 server and is certainly friendlier. Many Linux distributions include a configuration tool that is easier to use (Debian's dexconf) or autodetects most (if not all) settings (RedHat, SuSE and Mandrake chose this path).
It is also possible to use XFree86 in a framebuffer device.