Do we need a "Manhattan Project" for green energy?

Scientists are hard at work on renewable energy technologies, but which ones should we bet on? Clean coal, biodiesel, fuel cells, ethanol, nuclear, wind, solar cells made out of spinach? Should we just let the market pick winners, or should the government take charge? If such a project can happen, who will pay for it? Do all nations need to be involved for solutions to work? Who should, or could, lead the charge?