MultiMap is a company that sells maps and aerial photos, primarily for Great Britain. They offer an online map tool that is very similar to that used by MapQuest or Yahoo Maps.
However, for certain regions, they have this fabulous capability to show you the aerial photo of the place, then display hovering around the cursor a slightly transparent map. For instance, take a look at this map of the Tower of London and the Tower Bridge.
There are probably many more other innovative uses of this type of transparency overlay tracking to the cursor that can be helpful in other web tools.
[original link via Futurismic]