Tcl/Tk Graphical User Interface (GUI)
GRASS's Tcl/Tk graphical user interface is currently undergoing a complete rewrite using wxPython. It is currently available for testing in GRASS 6.3 and will be the default GUI in GRASS 6.4 and GRASS 7. See also these screenshots.
Not really a GUI, but loved by advanced GRASS users: the command line interface. Worth learning it!