It's a shadow as faked by a GIMP or Photoshop user. Although it's overused, it's a simple but effective means to enhance the sense of overall style of graphics on a page. Also, there's plenty of variations to be performed on it (for example, a translucent shadow-casting material, or even something casting a different-colored shadow). Just because something is used a lot doesn't mean it shouldn't be used any further; after all, a pixel is a pretty trite drawing primitive...