ProgrammableWeb has been tracking Twitter mashups since the first one was added in December, 2006. Looking at the number developers have added to the database, one thing is clear: 2009 was a huge year for Twitter and apps built upon it.
A year ago Twitter was just a microblogging platform. Sure, it was a popular one, but it wasn't until the announcement of its geotagging API that it took its first step toward being a location platform. Since then, it has expanded its offering to include the four geo APIs I've categorized below.