If your car is parked and you open the door and someone hits your door with their car who is at fault?

Technically it would be your fault as you are not supposed to put parts of your car into the path of a car. Having said this, no accident is ever 100% someone's fault. In Wisconsin you are 10% At Fault just for being there.

As the answer above stated, it is your responsibility to look for oncoming cars before opening your door. The only situation in which there may be an exception to this is if you were in a parking lot and the other car pulled in beside you after you had already begun opening your door.