Jeff's gets a no-prize rocketing down from the sky for bringing us this celestial impact calculator. Want to know just what would happen if, say, a rock the size of your car were to hit the earth? You'll find out here!
I ran a simulation with a 1 mile across dense object traveling at asteroid speeds hitting at a 35 degree angle on semi-porous rock. The result was a crater 15.3 miles in diameter. This was rather unfortunate for me, as I'd placed my virtual self only 1 km away. But boy, what a view!