Finally, there is an automated CSS Sprite Generator (via)! Just upload a zip of graphics and it will spit out a single image and a block of CSS you can use.

What are CSS Sprites? If you use a ton of CSS backgrounds for things like icons, buttons and other non-repeating graphics, then you can combine them into a single graphic and use background-position with width and height to show a slice of the larger graphic. This can make a site load faster since there are less files to download.

This is generally a pain to do by hand (not only combining the graphics, but calculating and remembering pixel positions), but with a tool like this it should be a cinch!