The Health team at Bing used public data to create a Health Map application (requires Silverlight) that lets users visualize a number of health indicators, such as obesity and premature births, by U.S. county.
Five years ago Paul Rademacher reverse engineered Google Maps to put Craigslist homes and rentals on a map on his site. The site essentially pre-launched the map mashup category, because there was no Google Maps API at the time. Now it is the prototypical example and still used by many to find their new homes. And, of course, Google Maps is now by far the most popular API to use.