The market's dramatic, headline grabbing upswing in acquisitions during the past few months is masking another key trend that often follows a merger wave: the return of the spinoff. For investors who crave adventure, no area is more fraught with extremes of profit or doom than businesses newly liberated from the grip of corporate giants. Right now the surge in the ranks of spinoffs is offering investors a choice of freshly minted companies in fields as diverse as biotech, natural gas exploration, and videogame retailing that, on paper, seen likely to profit mightily from their independence.