There are several protocol multiplexor tools which provide for the
simultaneous display of X clients on any number of machines. This ability to
echo one display onto another is useful for demo purposes, for teaching, and
for testing. Other related programs are useful as conferencing mechanisms;
they typically involve several parties, with the results of the electronic
conference visible on all screens:

XMX (an X Protocol Multiplexor) is a standalone utility for sharing an X
Window System session on multiple displays. XMX takes advantage of the
networked nature of the X Window System by acting as an intermediary between
X clients and X servers.  In this way, XMX works with any X clients and any X
servers, without the need to modify either.  XMX accepts multiple X client
connections, and displays client graphics on multiple X displays.  XMX paints
the same graphics on all servers, providing a WYSIWIS (What You See Is What I
See) environment.  An upcoming version is being developed. In the current
version, one machine controls input while all others are passive observers,
an arrangement which does not allow private work.  XMX version 1 multicasts a
complete X session (as opposed to individual windows) to any number of X
servers. The X server which provides input to the shared X client
applications (the "master") is established at setup time and remains fixed
throughout the session.  Similarly, the "slave" servers which participate in
the multicast are established at the start.  There is no facility for adding
or removing a server to or from the session.  The software is available at
ftp://ftp.cs.brown.edu/pub/xmx-1.1.tar.Z .  For more info see
http://www.cs.brown.edu/software/xmx/ . [6/96]

XTV is a conference program which can be used to duplicate the "chalkboard"
on several displays. Release 1 is available on the X11R5 contrib tapes; a
more recent version [7/95] is on ftp://ftp.cs.odu.edu/pub/wahab/XTV/ ;
binaries for several systems and source are there. Information:
wahab@cs.odu.edu. [6/96]

Shared X, a modified X library to give dynamic multi-display support, by
Michael Altenhofen (altenhofen@kampus.enet.dec.com), was
done as part of the NESTOR project being conducted at Digital CEC Karlsruhe
in collaboration with the University of Karlsruhe.  (The NESTOR project is
described in "Upgrading A Window System For Tutoring Functions", Michael
Altenhofen et al., in the proceedings of the EXUG Conference 11/90.)
It provides "naive" X applications with the capability to
work with several displays simultaneously, dynamically add and remove
new displays and hand input control from display to display.
shX was mainly intended to be used in a student/tutor environment,
where the tutor would need to see the students current work, make some
modifications to it on-line and would then drop out of the application
again. It may be generally useful for group work and presentations.
Sources are on ftp://ftp.x.org/R5contrib/ . [6/96]

Modifications to shX for color mapping and private color allocation by Mark
J. Handley (M.Handley@cs.ucl.ac.uk) are on 
ftp://cs.ucl.ac.uk/car/ .

XTrap is an extension to X that facilitates user emulation; it can be used to
record and then replay an X session.  Device input synthesis, device input
monitoring, and grapics request output monitoring are supported by XTrap.
XTrap comes with a client-side library which makes the new functionality
easier to use in an X application.  XTrap also comes with sample clients
which demonstrate the various capabilities of the extension and library.
Please see the man page within programs/xtrap/xtrap.man for more
information.  It is available as
ftp://ftp.x.org/contrib/extensions/XTrap_R6_v34.tar.Z .  (Note: the XTEST and
RECORD extensions in R6 provide input synthesis and protocol recording
respectively.  Taken together, they provide functionality similar to XTrap.)
[6/96]

The program "wscrawl" is a demo of the networking capabilities of X-Windows.
Wscrawl can be thought of as a paint program shared between multiple people,
displays, and workstations.  Any number of people can draw independently of
each other, yet they all work on the same picture.  The word "wscrawl" stands
for "window-scrawl". The user may think of wscrawl as a paint program shared
by any number of people at the same time.  When wscrawl is run, it opens up a
separate window on each participant's display.  From that point onward, each
participant sees the actions and ideas of every other participant as they
occur. Each individual may simply watch, or participate at any moment.  Any
individual may exit out of the session at any time without affecting the
other participants.  Sources are on
ftp://sunsite.unc.edu//pub/archives/comp.sources.x/wscrawl/ . [6/96]

Shdr (by Rank Xerox EuroPARC) implements a simple shared whiteboard, with
scribble-style drawing and text drawing.  The drawing surface is entirely
shared and entirely unstructured, being simply a bit plane. The mouse acts
either as a pen (in 'draw') mode or an eraser (in 'erase' mode). When no
mouse button is pressed, it acts as a pointing device; movements are echoed
on the remote screen.  It features continuous telepointing to both sides, so
there's no mode switch between marking, pointing and entering text.  There's
no chalk-passing mechanism; both sides can make/erase marks at once. The
drawing object is replicated on both sides, so that the network traffic is
reduced.  Sources are on ftp://parcftp.xerox.com/pub/europarc/shdr.tar.Z 1.3a
appears to be the final version. [6/96]

xmove is a pseudoserver (aka proxy server) which allows you to dynamically
move an X application between servers See The X Resource (Summer 1994) for an
article on the motivation for and construction of xmove.  Version 1.2f is
current [6/96]; sources are on ftp://ftp.cs.columbia.edu/pub/xmove/ .

Vartalaap is a multiparty multimedia conferencing system that works over Unix
sockets; the interface is based on XView.  It's available at
ftp://ftp.x.org/R5contrib/.

Collage is a synchronous collaborative data analysis tool for use over the
Internet.  Features include a shared whiteboard, screen capture/sharing, a
shared text editor, and data-analysis tools.  Sources are on
ftp://ftp.ncsa.uiuc.edu/UNIX/XCollage/Collage1.3/ .  

TeamRooms provides "shared spaces" on the Internet allowing groups to share
information.  These electronic team rooms provide places to meet in
real-time, or a common locale to leave information for other collaborators.
TeamRooms combines real-time groupware technologies such as shared
whiteboards, chat rooms, and customizable groupware applets with a persistent
work environment.  A beta 1.0 is at
http://www.cpsc.ucalgary.ca/projects/grouplab/teamrooms/ , for several
systems. [8/96]

GroupKit is a Tcl/Tk extension that provides facilities for building
real-time groupware applications such as shared whiteboards, meeting tools,
games, discussion tools, etc.  The package comes with over 30 sample
groupware applications/demos.
http://www.cpsc.ucalgary.ca/projects/grouplab/groupkit/ . [9/96]

GroupKit is a Tcl/Tk extension that provides facilities for building
real-time groupware applications such as shared whiteboards, meeting tools,
games, discussion tools, etc.  Version 3.2 was released 8/96. Sources are on
http://www.cpsc.ucalgary.ca/projects/grouplab/groupkit/ .

Also of use [commercial products not recently confirmed]:

X/TeleScreen is a commercial implementation of a "tee"ing program.
Information: info@nis.com.

Hewlett-Packard Co. has a commercial product, "HP SharedX" which works under
HP-UX currently on their 300, 400, and 700 series workstations and their HP
700/RX X Stations.  Machines receiving shared windows can be any X server.
HP SharedX consists of a server extensions and a Motif based user interface
process.  Contact your local HP sales rep. for more information.

IBM offers a commercial product.

Sun offers multi-user confering software called ShowMe.

InSoft (Mechanicsburg, PA, USA, 717-730-9501) offers multi-user conferencing
software called Communique. Version 3.0 is available on Sun and HP
platforms.

TeamConference is a product which allows real-time sharing of X windows.  For
more info: http://www.spectra.com/products/teamC.html .

[Thanks in part to scott@spectra.com (Tim Scott), 5/91, and to Peter Cigehn
(peter@lulea.trab.se), 8/92 ]