X11, or X, is a vendor-neutral, system-architecture neutral network-transparent window system and user interface standard. In other words, it's a GUI for UNIX. X can use your network -- you may run CPU-intensive programs on high powered workstations and display the user interface (the windows) on inexpensive desktop machines.
X.Org has taken over maintainership of Xaw3d since D.J. Hawkey's passing. The 1.6 release was the first made by X.Org: http://lists.x.org/archives/xorg-announce/2012-January/001800.html and a 1.6.1 bugfix followup has been made as well: http://lists.x.org/archives/xorg-announce/2012-February/001827.html