Free viewing is when you view stereo images without a viewer. It can cause headaches and may take some time to get used to. The parallel method and cross-eyed viewing are two methods of free viewing.
The parallel method is viewing a stereo image with its left image on the left side and right image on the right side. Most stereo cards are made using this method instead of placing the images for cross-eyed viewing. With this method you are restricted to images that are no wider than the distance between your eyes.
The cross eyed viewing method pairs the left eye with the right image and the right eye with the left image. It is mostly used with large images and is easier to learn than the parallel method.