The method that most of us use nowadays is to duplicate the layer, fill it with solid black, duplicate, and move it 1 pixel down and 1 pixel right (this is for a sun in the top left) to create a 45 degree shadow angle. Do this 10 times or so (10 layers) then merge the layers. You can do a 30 degree by moving down 1 and right 2. If your buildings have a stroke on them don't forget to increase the size of the selection by the same amount as the stroke.