Information about X11R7.1 can be found from the X.Org Foundation wiki, and at mirrors of this server.
This is the second release of the new modular source code tree. The source code has been split into nine logical modules: app, data, doc, driver, font, lib. proto, util and xserver. Each of these modules contain one or more packages that can be configured, built and installed separately. Please see an X11R7.1 release site for a complete list of the tarballs.
For information on how to build the modular tree packages see the Modular Developer's Guide. This guide also contains information for developers who want to help improve the modular build system and modular code base.
The X source code for this and all releases/snapshots as well as development versions can also be accessed via the Freedesktop.org CVS repository. It's also possible to browse the freedesktop CVS repository. The CVS tag for the 7.1 version is "XORG-7_1".
To check out the latest development version, don't specify any tag.