(FORTUNE Magazine) – If anyone on this list could go unnoticed in a roomful of big-name executives, it would be Ivan Seidenberg. What he lacks in star power, though, he makes up in market might. Seidenberg, 56, runs the nation's tenth-largest company, providing mission-critical phone lines to many parts of the U.S. government, the city of New York, and the New York Stock Exchange--not to mention dial-tone and cellular service to scores of VIPs all over the country. His influence extends far beyond telecom. When a feud between Viacom's Sumner Redstone (No. 17) and Mel Karmazin became public last summer, sources say it was Seidenberg, a longtime Viacom board member, who persuaded the executives to resolve their issues in private. That kind of power certainly should make a crowded room take notice. --Stephanie N. Mehta