Over the weekend I was playing with a split level neutral density filter. It slips into a filter holder on the front of the lens, and basically darkens one half of the image. That allows you to take shots where one half is much darker or lighter than the other (a classic example is shots that have some sky in them-- because on most days, the sky is much brighter than the rest of the image.) The filter works pretty well-- using the Cokin P-system, it can be attached and removed fairly quickly. And it really does help with certain shots. It does have its limits, however-- I was using a 2-stop split filter-- for situations in which the halves of the image are further apart than that, the filter doesn't help. For those situations, bracket three or four stops and do high-dynamic range combos in Photoshop.
Here's a complementary view, looking North.