About the X Window System
The X Window System is the glue that holds together the desktop on Unix and Linux systems. It provides the layer between the hardware device drivers and the desktops such as GNOME and KDE. It has been developed by a variety of industry consortiums and open source projects over the last two decades. The current maintainer is the X.Org Foundation, a non-profit group organized to oversee the open source community of X developers.
X in Solaris
Solaris currently includes software from two implementations of the X Window System.
The older version was once part of Sun's OpenWindows software and is still installed into /usr/openwin. It has evolved along with the X open source releases over the years. The last full merge of an X open source release into this tree was for X11R6, though large portions of the later X11R6.4 and X11R6.6 releases, and selected portions of the XFree86 4.3 and X11R6.8 releases, have been merged in more recently. This tree provides the Xsun server and a few X client libraries and applications still.
The newer tree is based on the X11R7.x modular releases from X.Org and is where Solaris gets the Xorg server and the libraries and clients now delivered in /usr/X11. We've been working to migrate all of the existing libraries and clients to build from the this source base as well for future Solaris releases, and have almost finished that now.
For shorthand, these are often referred to as the "Xsun" and "Xorg" trees, even though they are much more than just those X servers, but the X servers they build are the primary differentiator.
X in OpenSolaris
As explained in this "Opening Day" blog post, the initial phase of code release for OpenSolaris in June 2005 did not include either of the X source trees. We've now started releasing the already open source code from the X trees - you can find the list of which portions and the download links on our sources page. As we move more and more of the X components to being built out of this tree, the OpenSolaris X tree will cover more and more of the Solaris X packages.
Even though not all of the X source code isn't posted on the OpenSolaris.org web site yet, there are still ways to build your own version and even submit changes that could be incorporated into Solaris. You can download either the X11R7.4 release to get the same base Solaris is built on, or check out the X.Org git to get the base that will be used for future releases of Solaris. If you've got changes that are of general use, you can submit them to the X.Org community via the "xorg" product in the freedesktop.org bugzilla. Once there they can be pulled into the Solaris builds either as part of the next X.Org full release, or individually on an earlier schedule by one of Sun's X developers.