If GISCorps has already captured your attention, CartONG will also interest you.
CartONG is a French Non-Governmental Organisation (NGO) specialising in Information Management. Their mission is to put (geographical) data at the service of humanitarian, development, and social action projects, working towards improving the lives of vulnerable populations.
They do this 'humanitarian cartography' across four continents, and their services cover Capacity Building, Information Management, Mapping & GIS, and Mobile Data Collection.
Interested? Have a look at the projects [FR] they're currently working on and become a volunteer.