The window manager in X is just another client -- it is not part of the X
window system, although it enjoys special privileges -- and so there is no
single window manager; instead, there are many, which support different ways
for the user to interact with windows and different styles of window layout,
decoration, and keyboard and colormap focus. In approximate chronological
order (generally, the more recent ones conformant more with the ICCCM and
are the only ones being maintained):

wm: this simple title-bar window manager was phased out in R2 or R3

uwm: the Universal Window Manager is still popular for its speed, although it
is very outdated. Moved to contrib/ on the R4 tape.

twm (old): Tom's Window Manager was among the first non-Consortium window
managers and offered the user a great deal of customization options in a
re-parenting window manager.

awm: the Ardent Window Manager was for a while a hotbed for hackers and
offered some features (dynamic menus) not found on more current window
managers

rtl: Siemen's window manager tiles windows so that they don't overlap and
resizes the window with the focus to its preferred size.

dxwm: Digital's dxwm is part of the DECwindows offering

hpwm: HP's window manager offers a 3D look; it is a precursor of mwm

mwm: the Motif window manager is part of the OSF/Motif toolkit

tekwm: Tektronix's window manager offering

olwm (Sun): olwm implements the OPEN LOOK GUI and some of the Style Guide
functionality

olwm (AT&T): ditto

gwm: Bull's Generic Window Manager emulates others with a built-in Lisp
interpreter. Version 1.8 is in koala.inria.fr:/pub/gwm/ and on ftp.x.org
[7/95]

m_swm: the Sigma window manager is on the R4 tape

pswm: Sun's PostScript-based pswm is part of the OpenWindows release

swm: Solbourne's swm is based on the OI toolkit and offers multiple GUI
support and also a panning virtual window; configuration information comes
from the resources file. Sources are on ftp.x.org in R5contrib/swm.tar.Z; they
require OI binaries.

twm (new): the new Tab Window Manager from the R4 tape is a reworked twm and
is the basis for several derivatives, including the one on later X releases

vtwm: vtwm offers some of the virtual-desktop features of swm, with a
single-root window implementation.  A new version, vtwm-5.3, is based on the
R5 twm and is available from ftp.x.org. [1/94]

tvtwm: Tom's Virtual Tab Window Manager is also based on the Tab Window
Manager and provides a virtual desktop modeled on the virtual-root window of
swm. It is available on ftp.x.org and mirroring archive servers. The current
[3/95] version is available at
ftp.x.org:/contrib/window_managers/tvtwm.pl11.tar.gz.

olvwm: the vtwm-style virtual-desktop added to Sun's olwm. It is available on
archive servers; version 4.1 [2/94] is on ftp.x.org.

mvwm: the vtwm-style virtual-desktop added to OSF's mwm. A beta version is
floating around (most recently from suresh@unipalm.co.uk) but requires a
source license to OSF/Motif 1.1.3 [3/92].

NCDwm: the window manager local to NCD terminals offers an mwm look

XDSwm: the window manager local to Visual Technology's terminals is simple
but full-featured.

ctwm: Claude Lecommandeur's (lecom@sic.epfl.ch) modification of the R5 twm
offers 32 virtual screens in the fashion of HP vuewm and also offers the
window overview used in vtwm and tvtwm. Version 3.3 [9/95] source is on
ftp.x.org and possibly also sunsite.unc.edu.

vuewm: HP's MWM-based window manager offers configurable workspaces.  SAIC
offers a version of this VUE environment.

4Dwm: SGI's enhanced MWM

piewm: this version of tvtwm offers pie menus

pmwm: IXI's Panorama version of MWM offers olvwm-like features.  Info: +44
223 236 555, +1 408 427 7700; mmoore@x.co.uk or michaela@x.co.uk or
laurie@ixi.com.

fvwm: this virtual window manager has been rewritten from scratch and is very
light on system resources (between half and two-thirds the memory usage of
twm, on which it was based). fvwm offers most of the features others provide,
plus additional features. Source is available from sunsite.unc.edu in
/pub/Linux/X11/window-managers/; fvwm-1.24r-source.tar.z was current in 1/95;
2.1.0 is expected early 1996. Information:
http://neutrino.nuc.berkeley.edu/neutronics/todd/fvwm.html 
and ftp://ftp.hpc.uh.edu/pub/fvwm/version-2/ .

mwm 2.0: the 2.0 version of mwm includes support for multiple workspaces.

9wm, by David Hogan (dhog@cs.su.oz.au), is an X window manager which attempts
to emulate the Plan 9 window manager 8-1/2 as far as possible within the
constraints imposed by X.  The latest version of 9wm is held at
ftp://ftp.cs.su.oz.au/dhog/9wm .

mwfm: MWFM is a Microsoft-Windows-Program-Manager-style applications
manager.  It offers Unix users the ability to work in a MS-Windows-like
environment.  Sources are at ftp.x.org:contrib/desktop_managers/mwfm1.0.tar.Z.

Also of possible use is vr, by Richard Mauri (rmauri@netcom.com), on
ftp.x.org and ftp.informatik.tu-muenchen.de
(pub/comp/X11/contrib/clients/vr/vr-1.01.tar.Z); Vr is a workspace manager
intended to be window-manager-independent.