"What are you doing?" That's the central question behind Twitter, the uber-popular microblogging/social networking service--and, at this point, cultural phenomenon. In 140-characters or less--brevity's a virtue, just ask Jack Dorsey, Twitter's cofounder and CEO--users enter status messages about what they're doing, reading, researching, pondering, whatever; and others comment or post their own "tweets." If it sounds simple, or basic, that's because it is.

The true value of the service lies in the ability to tweet from wherever you are. Twitter.com offers its own mobile portal that you can access using many new mobile devices; you can setup up SMS tweet service, as well, to send text messages as tweets. But the explosive growth of Twitter has also led a handful of crafty coders to create their own mobile Twitter clients for a wide variety of devices, including the popular BlackBerry and iPhone, as well as Windows Mobile handsets.

Whether you employ Twitter for work, play or a combination of the two, there's a mobile Twitter app that's best for you. Which app that may be depends on your smartphone platform. Check out the following seven mobile Twitter apps for a look at your options.

Mobile Twitter Apps for BlackBerry

TwitterBerry

As loyal users of both BlackBerry devices and Twitter, we're very familiar with the mobile Twitter applications available to Research In Motion (RIM) smartphone users. Our personal favorite: TwitterBerry.

TwitterBerry's a free, easy-to-use mobile application for BlackBerry, and it offers much of the same functionality that the standard, desktop Twitter does. With a single click from the BlackBerry home screen, users can input Twitter updates. Replies to status updates, as well as timelines of their latest posts and the last public posts, are available through the TwitterBerry menu, which is accessed via the BlackBerry menu key. We even hear that the good folks over at Orangatame Software--namely Jason Schroder--are working on a feature that would automatically update Twitter streams and provide notifications--two extremely valuable options that are notably absent from TwitterBerry.

Our only real complaint about TwitterBerry: you must repeatedly return to the application's home screen to refresh friends' timelines, replies, personal timelines and more. This gets tedious quickly, and a way to refresh those individual pages without ever leaving them would be very welcome.