Sprite Cow is a unique tool with a convinient, easy to use interface for getting the details of CSS sprites.
You load a picture either from browsing for it or by dragging its file from the computer and dropping it onto the page. Then clicking on singualted areas on the image will automatically detect their boundaries and output the CSS code needed for using the selected area as a sprite.
The tool, made by theTeam, uses HTML5 Canvas for image proccessing, and can even handle non-transparent backgrounds. Try the Example Image to grasp its features easily.