A co-worker turned me on to this - betsyjohnson.com has a great interface for viewing details of the clothing they sell.
When you view the page for any item, for example this dress, and place your mouse cursor over the image, a grid appears behind the arrow, and a new image window appears at right that shows a dynamic closeup of where the grid is positioned over the main image.
This code may be in use in other places on the Web but this is the first I've seen it.
11/28/07 - The Gap has it too; it's mainstream. I still think it's cool.