Canon 550D, Tiffen 2 stop graduated screw in filter to help equalise sky/land light levels and just Canon's free Digital Photo Progression software to edit. I shoot raw most of the time and then convert to black and white in DPP, generally using the red digital filter then manipulate the various contrast controls. The best starting point though is always a dramatic sky in the first place, right under a storm cloud helps alot as you get both a dramatic sky and dramatic light coming in from the side underneth it to make the ground more striking.