I so need one of these Magnetic Pet Doors for my dog Mojo. She has no ability (or desire) to paw open a door so she barks to be let in or out if the door opening is less than twice the width of her body. With this system, you just put a small magnet on the pet's collar and then the door opens automatically as they approach and closes once they've stepped through. You can also limit access by setting the door to In Only, Out Only, or locking it entirely.
$59.99 at Smarthome.