The FVWM window manager is a light alternative to the bloated default Ubuntu desktop. This is a window manager like TWM, but with more features. Download a copy and get more information here: http://fvwm.org/.
TWM is the default window manager that ships with the Xorg distribution. This is a very simplistic window manager that has a root menu and is customizable with color schemes and themes.
Blackbox. This window manager is rather like Fluxbox, with the same support for themes and extremely light on resources. This is a good one to use if you are using an older computer and need something that is not demanding. Or if you want to use a computer to its full potential. http://blackboxwm.sourceforge.net/.
Olwm. This is a light window manager that implements various features of the Open Look Window Manager. This was once the standard window manager for SUN workstations before they switched to MOTIF/CDE and then to the Gnome desktop environment.
Icewm. A window manager which has a layout akin to Windows `95. This is a very fast WM and may be themed to look exactly like Windows XP. http://www.icewm.org/.
Afterstep. A NeXTSTEP-alike window manager for Linux and UNIX. This is a very pretty window manager and still looks very good today. http://the-labs.com/AfterStep/.
Metacity. This is a GTK based window manager that was integrated into Gnome before they switched to Mutter. This is still usable today though and there are many good themes still being created. http://ftp.gnome.org/pub/gnome/sources/metacity/.
Windowmaker. A NEXTSTEP compatible window manager for UNIX and Linux. This is very fast and agile even on a Pentium II. Well worth installing if you require a fast interface. http://windowmaker.org/.
CDE. Common Desktop Environment. This is a commercially licensed desktop environment that was used on older SUN workstations. This is now available as a source release and hopefully will be added to the Ubuntu and Linux Mint repositories. http://www.immt.pwr.wroc.pl/faq/msg00025.html. CDE FAQ.
Openbox. A lighter and simpler version of Fluxbox, with an emphasis on speed. The best minimal window manager. http://openbox.org/wiki/Main_Page.
Ratpoison. A light window manager that focuses on keyboard use and not the mouse. http://www.nongnu.org/ratpoison/.
AwesomeWM. This is a tiling window manager for UNIX and Linux. This is very fast and easy to use. http://awesome.naquadah.org/.