Twitter made the leap this year from tech-world phenom to mainstream brand. The microblogging platform allows users to write and disseminate short messages, known as tweets, that are capped at 140 characters. More than three million users worldwide have set up accounts, and nearly every major news event this year marked a surge in traffic. Twitter CEO Evan Williams posted updates on the company's progress -- and his own daily habits -- constantly. So did Barack Obama and Dave Matthews. As Inc. observed in a March profile of Williams, the company (which raised $15 million in 2008) hasn't figured out how to make money. Still, few businesses can claim to be as much a part of the Zeitgeist.