13. Carl Icahn, Chairman, Icahn Enterprises

What makes him powerful: Icahn, the billionaire activist investor and corporate raider, is one of the richest men in the world, with a net worth of around $14 billion. His investment strategy is simple: Target a company he believes suffers from poor leadership, start buying up shares of its stock and then raise serious hell until sweeping executive changes are instituted. It's an approach that's hit the jackpot time and again, because the more havoc that Icahn wreaks, the more the companies in his crosshairs seem to improve--and so does his bottom line.

In some respects, Icahn's increasing profile in the wireless industry is a ringing endorsement of the segment's economic prospects--if he didn't believe there is serious money to be made here, he wouldn't waste his time. Of course, if you're the CEO of one of the firms in Icahn's portfolio, you may wish he'd set his sights elsewhere. Consider what has happened to Motorola and Yahoo, two marquee companies significantly transformed by Icahn's efforts.

The most highly publicized tussle pitted Icahn against Motorola: After Ed Zander departed as CEO at the end of 2007, Icahn publicly championed a plan to split the firm into four parts. His holding company owns 6.4 percent of Motorola's outstanding shares, and he fought to become a director of the company, a move that shareholders shot down in May. However, in late March he sued Motorola for documents relating to its mobile devices business and to install his handpicked nominees on the handset maker's board of directors. In early April, Motorola and Icahn settled their dispute and all litigation, and the firm agreed to approve of two of his nominees, Keith Meister, the principal executive officer of Icahn Enterprises and William Hambrecht, founder, chairman and CEO of financial services firm WR Hambrecht and Co. Icahn also won authority to serve as a consultant on the future of the mobile devices division, including its search for a new CEO.

When Microsoft's proposed takeover of Yahoo went sour in the spring of 2008, Icahn launched a proxy fight and sought the ouster of the web services giant's CEO and co-founder, Jerry Yang. In July, Icahn and Yahoo settled the proxy battle, and Icahn won himself a seat on Yahoo's newly-expanded board. Far less contentious (at least so far) is Icahn's relationship with mobile content firm Motricity: In February 2007, Icahn poured $50 million into the company, and his son Brett joined Motricity's board of directors at that time as well.

The brief: When Icahn wants something, he has the money and the muscle to get it. As economic turmoil reshapes how the world does business, look for his crusade to grow even more fascinating.

13. Carl Icahn, Chairman, Icahn Enterprises