This website is about X and the Motix window manager with many links to other sites.
This website is an excellent Guide to the various X window managers available.
An article describing the X Windows system from a programmers point of view.
KDE is one of the most popular graphical environments used with X.
Their website states "The XFree86 Project, Inc is a non-profit organisation which produces XFree86, a freely redistributable open-source implementation of the X Window System".
A web site by Eric Foster Johnson who has written several popular Linux books.
Their website states "X.Org is the worldwide consortium empowered with the stewardship and collaborative development of the X Window System technology and standards".