I added a depth of field shader to Rivers, an effect I’ve wanted for a long time, since I really love how it looks in miniature / close-up photography. Here’s a test run with some blocks:

and applied to the usual scenery it lends a hazy atmosphere:

The technique records distance-to-camera values for each pixel (smuggle them in the alpha channel) and then interpolates between a sharp version of the scene and a blurred one as outlined in this nVidia paper and this post.