Flying Medical Service based in Arusha, Tanzania require just an FAA PPL. They operate 2xC206s.
Very interesting operation. They do regular rotations of clinics lasting a few days in remote areas - in which the pilot is an active participant (doing medical stuff). They do a a fair amount of their own maintenance (being a private op) which is another interesting aspect.
Due to the very low income of their work (partly fundraising, partly contributions from those they help) their pilots are volunteers - a food allowance and accommodation is provided. You won't lose money working with them, but nor will you save a single penny.
They'll put a lot of effort into you (airstrip training, kiswahili language, medical training, maintenance training etc) and in return expect you to stay at least 3 years. I don't know of anyone who has broken this commitment which says good things about a) the people who work there, b) the type of person they'll take on and c) the rewarding nature of the work.
Certainly not for the hour builder - but if you really want to get involved in helping people and have at least 3 years to offer it could be very worthwhile.
They have a website here.
FLYING MEDICAL SERVICE