Peter Singer's famous essay "Famine, Affluence, and Morality" (PDF here) argues that charity is a moral imperative. He makes a strong case. The quality of philosophical arguments against this demandingness objection are best summarized by SMBC:
The Cold Equations. Uncomfortable feelings aside, math is math. I've read disparaging sneers about utilitarian ethics, but maximizing the number of lives saved seems pretty straightforwardly good.
For a little more info on effective altruism: