I'm a sucker for great user-interface design ideas, especially if they succeed with what typically has poor UI design -- web pages. 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.