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.
The latest release of desktop Twitter app TweetDeck launched with a ton of new features, which includes showing geolocated tweets. If a tweet includes the location meta-data, TweetDeck shows a tiny marker icon. Click the icon and you get a map of the location.
Largely in response to learnings from terrorism-related events, there's a new protocol for first responders that involves their entry into buildings before they're fully cleared of any danger. Commanding a situation like that requires GPS-like 3D indoor mapping technology where no GPS exists.