While start-ups have played a crucial role in getting the green industry off the ground, the future will likely be dominated by large, sprawling conglomerates. Why? Green technology essentially involves revamping the physical infrastructure of the modern world: replacing coal-fired power plants with wind turbines, building homes from materials concocted in chemistry laboratories, and swapping out engines for electric motors. Established companies simply are in a far better position to muster the capital, technological depth, managerial expertise and factory capacity needed. Familiarity plays a big role too. Millions have flocked to play Farmville. You won't see the same sort of giddy enthusiasm for those installing high voltage power lines or sewage-to-drinking water plants. If the Internet boom was an under-30 billionaire, clean tech is a science teacher with a comb-over. With that in mind, here is my list of the top ten Green Giants, the companies most likely to produce, develop and promote the ideas an products that will have the widest ranging effects.