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. Note that XFree86 made a license change around version 4.6. Many distributions have switched to http://x.org because this license change was incompatible with other licenses. The license change was the straw that broke the camel's back. Most of the X community has switched to x.org because it was felt there was more community participation. Rapid changes have started to occur in the x.org code for this reason! XFree86 is actually a play on words. It stands for X Three 86 as in the 386 Intel microprocessor. You can find the story some where on this site http://www.linux-mag.com/.