Is the taxi company liable for damages if its passenger in a parked taxi cab opens his door into the door of a parked car and dents it?

Yes. The taxi driver should have parked in such a way as to not impede the exit of the passenger. Taxi companies carry lots of insurance for obvious reasons. Which is also why the fares are usually costly. It could go both ways, if you didnt think you had enough room on one side, why not try the other side? It depends upon the damage and where the taxi cab driver parked. The law "should" seem fair and reasonable under the view of the average person. For example, if the driver parks so close you don't enough room to exit comfortably then he would probably be At Fault, but if there's no one next to you then someone pulls up and you still open the door without noticing the car next to you then you would be negligent. It depends on the circumstances. Basically you need to ask yourself who the idiot was, the driver or the passanger.