This is CDE running on a Sun/Solaris 7 machine.
(CDE is probably my favorite WM). CDE has been
accepted by all of the major UNIX vendors as
the
default window manager for their systems.
(note: Staroffice running on this machine, no more M$! :)
This is Openwin 3 running on a Sun/Solaris 7 machine.
It's use is now depreciated (it originated in the
mid
1980's on the SunOS 4.x machines.)
This is Gnome/KDE running on a Linux machine.
It is the current default for RedHat 6.x.
FVWM95
This is a mod to FVWM2 with a look and feel of Win95.
It's running on a Redhat 5.2 machine.
This is Twm which is a MWM like window manager
running on a Linux machine. (Motif Window Manager
is really old, few people use it anymore.)
However, as Michael from Germany pointed out, it is still used by
MI/X. It is also the default window manager for VNC.