Interactive maps have been one of the hottest tools (trends?) in online real estate. You have Zillow, Trulia, and others that offer capabilities beyond what you would probably do on your own. Well, I'm a Type E and am always working to implement new and exciting ideas, especially maps.
Something that I've been working on has been a way to take city, county, and utility info to map service boundaries and availability. Thanks to Real Estate Tomato, I came across a site that has some of the ground work already laid. Nothing pretty, but very functional. Check out the boundary maps.