treewm is a window manager that tries to implement a new concept. In addition to
the client windows the user can create desktops which can themselves contain
windows and desktops. By arranging the windows in such a tree the user is able
to manage his tasks efficiently.

More info: http://www.informatik.uni-frankfurt.de/~polzer/treewmdurchnull/

WWW: http://treewm.sourceforge.net/