"Selling mosquito nets cheaply in malaria-afflited areas is more effective than giving them away, because people who pay for them will value them more and be more likely to use them."

Plausible, but false. I got it from this New Yorker article about randomized trials of social policy, which might have other good nuggets for you as well. (The economist doing the studies annoyed some people by showing evidence that microloans don't work, or something like that--I read it in the hard copy a while ago, and the online version is behind a subscriber wall.)