Jack Dorsey

It was Dorsey who shared the world's first tweet, "just setting up my twttr," and conceived the idea to share a brief "status." Now, as Twitter readies its monster IPO, Dorsey is the only cofounder of the initial trio (Biz Stone, Ev Williams, and him) who still works at the company. Dorsey had moved on to found Square, the vaunted mobile-payment system that now processes $15 billion annually (where he's CEO), but in 2011 he added the role of Twitter executive chairman and still helps drive big decisions. (See Jack Dorsey: The pride of St. Louis.)