From Wikipedia:

In computer graphics and cinematography, high dynamic range imaging (HDRI for short) is a set of techniques that allow a far greater dynamic range of exposures than normal digital imaging techniques. The intention is to accurately represent the wide range of intensity levels found in real scenes, ranging from direct sunlight to the deepest shadows.

Basically, by merging several images with different lightning you can get a much deeper, colourful picture. This is a classic example. The three small pictures at the top are merged to produce the one below:

There is a group in Flickr devoted to HDRI. I've sifted through most of its 1000 HDRI pictures to bring you some that are worth noting (if I ever again see a picture of a sunset I will kill myself):

By clicking on the picture you'll be taken to the Flickr page it came from, where you can access a larger sized picture.

To find out more about HDRI, see this discussion in the HDRI Pool.