David Einhorn

Greenlight Capital President 1996-present

Then: As a short seller, Einhorn makes money by betting on stocks he thinks will decline. He became one of the most prominent people to short Lehman during the summer of 2008, which, when made public, put the firm on even shakier ground as it faced mounting losses.

Now: Einhorn, 40, made new headlines when he said in early September that a court ruling involving debt ratings could be "devastating" for ratings companies Moody's and Standard & Poor's, a unit of the The McGraw-Hill Companies.