In the orignal - tricky to very hard to correct that. See with digital cameras if the camera gets an overexposed or underexposed area (white or black) it records only white or black in those areas - no other details are logged. Thus you can't fix those broken areas in editing.
Shooting and editing in RAW mode can give you more leaniancy with how much you can deal with an under/overexposed element in a shot, but the best thing is a correct exposure at the time of shooting. Also if you read up and use your cameras histogram whilst in the field you can tell where over and underexposed areas are on the LCD and decide if you need to shoot again with altered settings. In the shot you have here (for example) you could counter the overexposure by shooting with either a faster shutter speed or a smaller (bigger f number) aperture (or a combination of both).
The only thing I can think of is a grad ND filter that will darken (for lack of a better term) the sky. The top left corner will still be overexposed unless you can find a grad ND filter that only darkens one corner of the filter!
Also, circular grad ND filters are not too great because they always put the transition in the center of the frame. You'd be best off with something like this. A cokin filter holder will free up your left hand too.
(I hope I'm not breaking any forum rules by posting a picture that's not mine!)