The company: Atebits, a Philadelphia-based software start-up that makes a $2.99 iPhone application called Tweetie

The idea: Twitter doesn't charge users to send and receive messages, but that doesn't mean people won't pay for an app that makes it easy to use on the go. Atebits's 25-year-old founder, Loren Brichter, set out to build a Microsoft Outlook for the 140-character set. "Twitter is like e-mail," Brichter says. "The vast majority of people use it for superficial crap, but a small percentage use it for something useful and powerful." Brichter put the first version of Tweetie up for sale in November 2008. To promote it, he sent a message to his friends on -- you guessed it -- Twitter.

The result: Tweetie is the most successful iPhone app for Twitter and has been purchased by 12.7 percent of all Twitter users, according to When Brichter released the most recent version, it hit the top of Apple's bestseller list within 36 hours, ahead of popular titles such as Electronic Arts's Madden 2010 and CNN's iPhone app.

How Twitter applications work: Twitter allows anyone to create and sell software that incorporates, organizes, and rearranges tweets in real time -- for instance, by collecting messages written by professional athletes (an app called Twackle) or translating tweets into the language of the high seas (an app called Post Like a Pirate). So far, Twitter has stayed out of the app business, preferring to let software developers and entrepreneurs add functions as they see fit.