The MIT Software Center ships the X Test Suite on tape.

A multi-threaded version of Xlib based on X11R5 patch 12 is now available for
anonymous FTP from (new version 1/93):

        - DEC on gatekeeper.dec.com (16.1.0.2) in /pub/X11/contrib/mt-xlib-1.1
        - the Consortium on ftp.x.org in /R5contrib/mt-xlib-1.1

Note that this source code did not become the Xlib used in X11R6, although
the Consortium made Xlib thread-safe with that release.

HP has made available drivers to permit the building of the X11R5 sample
server on the HP 9000 Series 700 workstations; the files are on ftp.x.org in
/R5contrib/R5.HP.SRV/. [8/92]

The Edinburgh University Computing Service and European X User Group have
created an on-line index of public domain X software. The index is available
through gopher and provides an index of the ftp.x.org/contrib archive, the
comp.sources.x archive and various X software found around the internet. The
service holds manual pages, README files , etc which can be browsed through.
A keyword search of the manual pages is also provided.  Information:
xindex@castle.edinburgh.ac.uk.

User-contributed software is distributed through the newsgroup
comp.sources.x, moderated by Chris Olson (chris@imd.sterling.com); also check
that group for posting information.

Richard Hesketh (rlh2@ukc.ac.uk) has been creating a list of freely-
available X sources.  The list is stored on ftp.x.org in contrib as
x-source-list.Z. It lists the main storage locations for the program and
international sites from which it may be ftp'ed.

The machine ftp.x.org has a great deal of user-contributed software in the
contrib/ directory; a good deal of it is present in current or earlier
versions on the X11R3, X11R4, and X11R5 contrib tapes. There are also
directories for fixes to contrib software.  The file on ftp.x.org in
R5contrib/0ftpxorg.dir.Z is a quick overall index of the software in that
area, provided by Daniel Lewart (d-lewart@uiuc.edu).

These sites used to and may still mirror ftp.x.org and are of particular use
for Australasia: Anonymous ftp: ftp.Adelaide.EDU.AU; ACSnet Fetchfile:
sirius.ua.oz.

The material on giza.cis.ohio-state.edu, which tends to duplicate the
ftp.x.org archives, is also available via anonymous UUCP from osu-cis, at TB+
and V.32 speeds.  Write to uucp@cis.ohio-state.edu (same as osu-cis!uucp) for
instructions. [the archive is now maintained by Karl Kleinpaste]

A new west-coast UUCP X11 Archive is administered by Mark Snitily
(mark@zok.uucp) and contains the full X11 distribution, the XTEST
distribution, an entire archive of comp.sources.x and other goodies.

The machine zok has a TB+ modem which will connect to 19.2K, 2400, 1200 baud
(in that order).  The anonymous UUCP account is UXarch with password
Xgoodies.  The modem's phone number is 408-996-8285.

In addition, UUNET Source Archives (703-876-5050) tracks comp.sources.x and 
provides 800MB+ of compressed programs on 6250 bpi tapes or 1/4" tapes.  It 
also mirrors ftp.x.org/contrib in its packages/X directory.