Joann Barkan has an excellent piece, "Plutocrats at Work: How Big Philanthropy Undermines Democracy", in the Fall Issue of Dissent.
Because they are mostly free to do what they want, mega-foundations threaten democratic governance and civil society (defined as the associational life of people outside the market and independent of the state). When a foundation project fails—when, say, high-yield seeds end up forcing farmers off the land or privately operated charter schools displace and then underperform traditional public schools—the subjects of the experiment suffer, as does the general public. Yet the do-gooders can simply move on to their next project. Without countervailing forces, wealth in capitalist societies already translates into political power; big philanthropy reinforces this tendency.