(A curious thing: the pointer to Quartz Extreme on the page points to this UK page (note the URL). A hint as to the origin of the new X11 environment?)
This is cool. I was never happy with XDarwin; I couldn’t figure out how to get it running again after upgrading to 10.2, it put crap all over my filesystem in directories that weren’t visible through the Finder, and it offered whatever window manager you wanted, as long as it was an ugly old-school non-Aqua window manager. The new release also lives happily as a little package file: X11.app.
Oh yeah: for those who don’t know already, X11 is a standard GUI toolkit for Unix. A lot of graphical Unix apps, including the Gimp (a Photoshop competitor) and the original Mosaic browser, require X11 to run. With this release, Mac OS X has an even firmer claim on being able to run almost every desirable piece of Unix software.