Malaria is a terrible infectious disease transmitted by mosquitoes. It causes untold suffering around the world, with about 220 million people infected in 2010 according to the World Health Organization (WHO). There's some progress being made against it, but as you can see in the graph above, there's still a lot of work to be done, mostly in Africa and, to a lesser extent, South-East Asia.
Those of you who live in currently malaria-free places, take a moment to appreciate how lucky you are. And you might not know this, but malaria used to be much more widespread around the world. Here's a map from the World Bank Development Report that gives you an idea:
If you want to do your part to help, there are all kinds of NGOs working to fight malaria. Some examples are: Against Malaria Foundation, Africa Fighting Malaria, Malaria No More, Together Against Malaria, Malaria Foundation International, Infanta Malaria Prevention Foundation, etc (this is not an exhaustive list, obviously).
The Gates Foundation is also doing a lot on malaria, but they have their own money and don't take donations...