Second most overused graphical design element found on web pages today. Usually created by duplicating the object to be shadowed, coloring the duplicate to dark gray or black, blurring the heck out of it with a good Gaussian blur, then placing the original back over the shadow at an offset.

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...