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.
Evan Jacobs is one of the developers doing some really interesting things with the Twitter API. Instead of using Twitter as a means of broadcasting information, Jacobs' apps are actually gathering information from Twitter to turn it into something more useful.