I tend to have another level of nested windows inside applications, also mostly full- or half-screen. It's at that level that sometimes other shapes become interesting, for example for a time slider (long and horizontal), or a list of files (narrow and vertical).

If only the world had evolved into one where operating systems could interact with applications in a way that would allow persistence of window layouts, scripting of user interface elements, and allowing a user to modify application layouts to their own preferences.