This is an awesome thing offered by the National Park Service. If you are permaneantly disabled then you are eligible for this pass. We were offered this by a National Park Ranger and it has really been useful. You can use it at any National Park and several other places, their website lists all the locations eligible.
We have used this Access Pass at several National Park around the country including Castillo de San Marcos National Monument, Muir Woods National Park and the Grand Canyon National Park.
Here is the link for more information: Access Pass