Sprites
Sprites are a fantastic tool that web designers should use in order to make their website better. In fact, if you load the Yahoo homepage, you'll be treated to a sprite extravaganza. They use a sprite that is 420x1700! While the file may sound huge, it's actually mostly a transparent gif taking only 20k of the site size and it houses nearly 80 images. Think about it, they just cut down on almost 80 URL requests!
Links to more sprite information: