Below are some amazing pictures that look 3D without you having to wear any special glasses!
How does it work? The most significant factor which contributes to depth perception is bionocular disparity–it’s the fact that your left and right eye, because they’re separated by a few inches, see a slightly different picture of what is in front of you. Your brain integrates these two pictures into a coherent vision but it uses the difference between the pictures to give you a sense of depth. To see what I mean look away from your computer screen across the room. Alternate closing one eye than the other; notice both the difference between the pictures from each eye and also that with one eye closed the world seems flat.
Most 3D pictures achieve their effect by showing a slightly different picture to each eye. The old fashioned red and blue glasses did it by color filtering; the modern day 3D movies use oppositely polarized lenses.
Below is a new technique I’ve never seen before: by alternating the left and right eye image fast enough your brain integrates them to give you a sense of depth.
[Claud pointed out in the comments below that these images work equally well with one eye closed, so it's probably not binocular disparity but instead motion parallax that causes your brain to perceive depth. Motion parallax is the change in relative position of objects in the foreground and background as you move. Lean left and right as you look across the room and notice how the objects nearest to you change position relative to objects further away. Basically, the images below are simulating movement even while you're sitting stationary at your screen tricking your brain into perceiving depth.]
HT to abduzeedo