UNC Global has a few award programs that support international internships. The CV Starr and International Internship awards applications are due March 18, 2011. UNC Global Health sponsors summer internships as well.
Partners in Health has a summer internship program. This is the NGO founded by Paul Farmer, a leading social justice advocate.
Here are a few other opportunities to check out (though I encourage a critical perspective about large international NGOs; see critiques like this one):
Save the Children (use job search tool with an “internship” filter)
SOS Children's Villages