Activist investing takes places in even the largest multinational firms, as was the case with Bill Ackman of Pershing Square Capital Management, who forced McDonald's (MCD) to make vital changes to its menu, improve customer service and realize hidden real estate value, all of which ultimately led to a doubling in McDonald's stock price.

Another large multinational firm engaged with an activist investor is Yahoo! ( YHOO). Now-board-member Carl Icahn has so far been unsuccessful in his attempts to get Yahoo! to sell its search business -- or even the entire company -- to Microsoft ( MSFT).

But sometimes the activist investor isn't just another hedge fund. Take GSI Group ( GSIG). Stephen W. Bershad, the CEO of Axsys Technologies ( AXYS), put his own capital on the line when he recently bought 3.3 million shares of GSI Group in the open market.

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