Social Enterpreneurship seems to be very in right now. Google has launched Project 10 100 (pronounced "Project 10 to the 100th", aka "googol" - what should have been the company's name), which is a call for ideas to change the world, in the hope of helping as many people as possible. They are committing up to $10 million (USD) to fund 5 ideas.
This should be a big idea which a huge reach. Read more and apply.