The photos on this site are in a format that requires cross-eyed viewing, called Free-Vision Fusion. It sounds worse than it is, and most people can view 3D pictures in this way (unless you have been cross-eyed before or have some other handicap involving one of your eyes). Some people need more time than others to perfect the mechanism, but once you have done so, it is really easy to do.

Free-vision fusion requires patience and practice because your willpower and eyes must do the work of the stereo viewer. You must be able to focus where your eyes aren't looking.

Place the image in the center of your screen.
Sit at your normal distance.
Slowly cross your eyes. You will see a double image.
Continue to cross until the middle two images overlap.
Adjust focus on middle image, keeping the two images overlapped.
You should see the image in 3D.

If you are having problems with this method, try the following alternate method:

Place index finger between images at bottom.
Focus on your index finger.
Slowly bring your finger towards your nose, staying focused on your finger, but paying attention to the background images in your peripheral vision. You will notice that instead of two images, there are four images floating about.
Continue bringing your finger closer to your nose- you will see the two middle images moving towards each other.
When the two middle images are aligned, or are on top of each other, stop moving your finger. You will now see three images in the background. The middle one contains the left/right images overlapped.
Slowly remove your finger from your field of vision, while keeping the middle two images aligned.
Gradually force your focus out to the combined left/right image in the middle.
If you have problems keeping a lock on the middle image, try leaving your finger in the image for a while, but still focusing on the center image. Use your finger to regain left/right fusion.
If at first you fail, don't worry. It took me a while. Relax, go away and rest your eyes, and try it again later.

For more info about how to take and view stereo images, see:
http://www.3dphoto.net/stereo/stereo.html
(From which this write-up was stolen)