OpenAndroMaps is a collection of OpenStreetMap based offline maps mainly for outdoor use (hiking, mtb) with emphasis on topographic details (heightlines etc) in a compact vector format implemented using the Mapsforge library.
The maps are rendered by many Android apps (OruxMaps and Locus among the better known ones). Under iOS the maps are for example rendered by Cartograph. A portable desktop viewer written in Java like Cruiser can also be used. See Mapsforge for more apps supporting it.