Dual Garage Laser Parking by Maxsa
I've used the ball and string solution in the past, but I didn't want the ball dangling down in the middle of my new garage (oversized).  This product is excellent.  Its accuracy helps you park exactly on the best spot to make sure the car is inside while maximizing the space in front of the vehicle for storage and other things (a workbench for example).  Installation is a snap.