The preconfigured Geocoding services are the following:

 OSM_NOMINATIM: http://wiki.openstreetmap.org/wiki/Nominatim
 
 MAPQUEST_NOMINATIM: http://open.mapquestapi.com/nominatim/
 
 GEONAMES: http://www.geonames.org/export/geonames-search.html

Visit the websites for information and usage policies.
