As someone who's survived cerebral malaria, my favorite charity is by far the Global Fund, which finances treatment and research for the three most devastating diseases today: malaria, TB, and AIDS.

Other great charities are the Hunger Project, which tries to overcome poverty by ensuring sustainable development, and Human Rights Watch, which monitors human rights abuses throughout the world. (In that line, there's also Amnesty International.)

Two more great ones are the International Red Cross, which has done some great work in Rwanda, and Medecins sans Frontieres, whose doctors I have the utmost respect for -- I've met several of them while travelling, and each had some remarkable (and often harrowing) stories to tell.

Finally, I'd be remiss if I didn't also direct you to the Cystic Fibrosis Foundation.