HideUI is an extremely simple command-line addon to temporarily hide/show specific elements of the UI.
the hidden frames will re-appear when you cross a zone or relog
-- I will implement a saving option eventually
commands:
/hideui -- hides most of ui and windows
/uioff
/showui -- shows hidden ui elements and windows
/uion
/hidexp -- hides xp and various bottom frames
/showxp -- show the xp bar
/hideab -- hides all 4 actionbars
/showab -- shows all 4 actionbars
see also: UI Cleaner -- has similar functionality to mine

