XFree86 is an implementation of the X Window System. It was originally written for Unix-like operating systems on IBM PC compatibles and is now available for many other operating systems and platforms. It is free and open source software under the XFree86 License version 1.1. It is developed by the XFree86 Project, Inc. The lead developer is David Dawes. The current version is 4.8.0, released December 2008.
For most of the 1990s and early 2000s, the project was the source of most innovation in X and was the de facto steward of X development. Until early 2004, it was almost universal on Linux and the BSDs.
In February 2004, with version 4.4.0, The XFree86 Project adopted a license change that the Free Software Foundation considered GPL incompatible. Most Linux distributions found the potential legal issues unacceptable and moved to a fork from before the license change. The first fork was the abortive Xouvert, but X.Org Server soon became dominant. Most XFree86 developers also moved to X.Org. The last XFree86 CVS commit (as of November 2012) was made on February 2009.
Read more about XFree86: Usage, Architecture, Release History