Here is a neat trick that has gotten quite some attention after Crytek took it under their wings back in 2007. It’s called Screen Space Ambient Occlusion (SSAO). Using SSAO correct will present the images with a shading model which relies on the current view port. 

Basic steps:

1. Take the depth buffer of the current frame
2. For each pixel in the buffer calculate the delta depth values of the surrounding pixels
3. The output of step 2 is stored in a new texture
4. Apply some 2D blur to make it prettier
5. Blend it over the current frame (perhaps using a multiplicative blend mode)

Et voila. 

Image 1 - the input depth buffer (found on google images):

Image 2 - generated SSAO ready for blending