Over time driveways can collect all kinds of stains. Page 179 in The Trouble With Tulip has some great ideas to remove these marks.
Step One - Wet down the stains with a hose and then, believe it or not, sprinkle them with lemon Kool-Aid! Cover them with plastic, let them soak for about fifteen minutes and then scrub with a brush and rinse. The stains should be gone.
But,if they're not -
Step Two - Get some oven cleaner, spray it on kind of heavily, let it sit for fifteen and then rinse.