We all know there are tons of urgent problems in the world, but we rarely try to prioritize them. Take a look at this video below (it's about 15 minutes long, but well worth it), where Danish economist Bjorn Lomborg (from the Copenhagen Consensus) asks "if you have a few billion dollars, which problem would you solve first...AIDS or climate change?", and prioritizes them purely from an economic perspective. Which problem is economically the easiest to solve (not just in terms of cost, but in terms of "return on investment"?).
Well worth your time.
I pretty much agree with their consensus, though I feel very, very strongly about many of the problems (like climate change). But I would really like to see how they estimated the costs. I would guess that they used typical economic measures (and so would have missed a lot of intangible, indirect costs that would occur with things like loss of forest cover etc).
(On a related note, you might want to read an old post of mine, The economics of conservation).