I think I may have already responded. But if not, it is
http://www.aircareinternational.org/
They travel to a South or (forgive me) Latin American destination 1-2 times/year. In March they went back to Costa Rico.

Hello Sean, I was very interested in reading about your experience in Brazil for 10 days. I've been to Brazil a few times, but now that I'm a nurse I am interested in doing some volunteer work there. I was wondering if you could tell me the name of the organization you went with. Thank you. Ash