I use cat litter, seems to work as well as oil dri for about an eighth of the price. Whatever is left i have had good luck with dishwashing liquid and a strong spray from the hose. I have used simple green too and had about the same results. If you have something really stubborn and a nice new concrete driveway, I bet some hand cleaner like D&L would work if you get the spot really dry before trying it. But those oil spots add some character, don't you guys think?
I let degreaser (The Zep puple stuff Poser mentioned is good) soak in, then I get out the pressure washer, that will get all but the stain off. I've had some very large oil stains (from the GMC NOT the 40 ) and this works well.