The MIT Poverty Action Lab has a fantastically simple, compelling list of seven ways to help achieve the Millennium Development Goals. I'm going to print these and put them in my wallet.
My favourite finding is still that deworming kids in Kenya at 50 cents each adds a year to their schooling. It's widely known in the academic community, but not enough outside it (and are there any case studies outside Kenya?).
My nomination for an eighth high-impact way to spend money is REDD: Reducing Deforestation and Forest Degradation. If done properly this could make a big contribution to carbon emissions cuts and help improve the productivity of smallholder agriculture at the same time. I have high hopes that this will form part of whatever deal emerges at Copenhagen; paradoxically, it may be easier to get it through if the rest of the summit is a flop, because the world will be desperate for some good news (though beware countries who think they can buy their way out of climate change on the cheap. We still has to replace those coal fired power stations with something better) . See here for a new Economist article about it.
What doesn't make the list? Stimulus packages, the war in Afghanistan and bank bail-outs. Personally, I think all three of the above are necessary to avert worse disasters (after all a global economic collapse would also cut the amount we can spend on development) but the ease with which we shovel vast amounts of money down the banks' throats is still staggering.