Although the computer screen is two-dimensional, today most users of
windowing environments control their systems with a one-dimensional
list of choices -- the standard pull-down or drop-down menus such as
those found on Microsoft Windows, Presentation Manager, or the
Macintosh.

An alternative user-interface technique is "pie" menus -
two-dimensional, circular, and in many ways easier to use and faster
than conventional linear menus.  Pie menus also work well with
alternative pointing devices such as those found in stylus or
pen-based systems.

piewm is a virtual window manager based on tvtwm, which uses pie menus.