Mars Curiosity 3D images by crossing your eyes. (opens in new tab)

The exact image I linked to is of particular interest. Here you can see something in 3D that you couldn't possibly tell just looking at the originals - that you're actually looking off a cliff edge. The landscape seen past the horizon of the cliff edge is clearly much farther off when viewing the combined image.

Here's a copy in case the original site gets overloaded: http://imageshack.us/a/img829/3783/t6cftimgur.jpg

For anyone having trouble seeing these, the trick is to sit fairly far back from your monitor, and cross your eyes enough that you see 4 duplicate images (each of the two images doubled up). You then lessen the amount your eyes are crossed (or bring yourself nearer to the screen) until the 2 inner images of the 4 are perfectly overlapped, such that you now see 3 duplicate images. If you can then bring your eyes to focus on the middle image, it will appear in 3D.