If you take two photographs from slightly different positions (the width of your eyes from each other), you end up with a stereo pair. This shows you a scene from the viewpoint of each eye. If you stare at these you can cross your eyes so that the right eye sees the left picture and the left eye sees the right picture. An easy way to do this is to hold a finger upright in front of the pair and look first at your finger, then look past it at the stereo pair. Tilting your head sometimes helps if you're not perfectly level. If you can do this, you'll see a third picture between them which is in glorious three dimensions!
WARNING: Don't do this for too long, as it may strain your eyes.
The pictures below are quite small, but if you click on them, you'll see a much larger version you can use to fill your screen for a better 3D effect. In IE F11 will make the viewable area bigger still. Enjoy!