I get this periodically, especially if the car has been jacked up from the front. It's just water with some black stuff, presumably from inside the exhaust. I initially worried about it a lot, then concluded its condensation building up in the exhaust, especially if you just run your car in the garage and don't actually drive it. Our local Z guru preaches to actually drive your car rather than just cranking it so you can get this water out of the system before it rusts things.