X.Org is the official organisation dedicated to maintaining the existing X Window System code base and engineering appropriate enhancements driven by current and future market requirements. X.Org periodically provides official X Window System update releases to the general public free of charge. X.org’s ftp site contains full sources for X11R5 and X11R6 as well as contrib areas and documentation. X.Org is a sub-organization of the Open Group.
It is unclear how active this is nowadays, given that the Xfree86 project has rather taken over X11 maintenance.