I would have trouble choosing between UNICEF and the World Wildlife Fund.
UNICEF, the United Nation's Children's Fund or something like that, is an amazing organization that helps children all around in the world in terms of survival, children's rights, etc. Their site is
http://www.unicef.org/.
The World Wildlife Fund is obviously a hardworking organization for the conservation and protection of the environment, endangered species, etc. Their site is
http://www.wwf.org/
I feel somewhat like I'm shamelessly advertizing something, but meh. I strongly advocate either organization; they're both for worthy causes.