A new breed of applications that can be accessed via mobile shortcode seem to be popping up with increased frequency. Thanks to the folks from SkinnyCorp who made Extra Tasty, we've started calling them Shortcode Applications or Shortcode Apps.
Twitter can be included in this category because technically you never have to visit Twitter.com to use the service. However, Twitter is also a full web application. We throw Instant Messanger in the mix too—it's definitely integral to the whole scene.
Oh, and speaking of such things: Twitter won Mashable's Top pick for Top Social Networks of 2006 in the Mobile category. Nice!