Stereo photography
The trick to be able to see depth in stereo photos is to look cross-eyed at them so that you more or less see three images, where there is a third one in the middle with an overlap. Keep your eyes at this position where you see a third picture in the middle. After a while your eyes will have adjusted focus and you'll be able to see the stereo photo with depth in it. It takes a while to get the hang of this technique, but, once you have succeeded, it is really intriguing to look at stereo photos. More information about stereo photography can be found here.
Stereo photos taken with a single camera (I)
Stereo photos taken with a single camera (II)
Stereo photos taken with a single camera (III)
Stereo photos taken with a double camera
Synthetic stereo photos created from focus stacks (animals) (I)
Synthetic stereo photos created from focus stacks (animals) (II)
Synthetic stereo photos created from focus stacks (other stuff) (I)
Synthetic stereo photos created from focus stacks (other stuff) (II)