Singh Ray Reverse ND filter.

I actually use the Hitec versions (due to the costs) and I mostly pull out the 3 stop. What I've noticed, at least on my 1.3 crop cam, is the image circle doesn't make effective use of the reverse grad towards the top. So effectively, I've got a slightly more expensive 3 stop hard.

FYI, the Hitec reverse grads are about $150.