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.
Seattle based Front Seat software launched a new service to rank public transit for an area. The service almost perfectly illustrates the value of open government data. The "Transit Score" offers a handy location score that's compelling because transportation is a major factor in picking an affordable place to live.