I volunteer for a charity called MapAction. They are a disaster mapping charity and most recently responded to the Earthquake in Haiti in January where maps were extremely important in the initial search and rescue phase and subsequent rehabilitation of the country.
MapAction are only some 30 deployable volunteers. My most recent mission was to Niger in December to train NGO staff in the use of GPS and Google Earth to map and monitor water boreholes. See more about this in an upcoming version of Geomatics World