Example of using OpenStreetMap with iOS 7+ MapKit
If you want to support something similar with iOS versions before 7 check out http://wiki.openstreetmap.org/wiki/OSM_in_MapKit
Descriptions of the various OSM versions of tiles available can be found here http://wiki.openstreetmap.org/wiki/Tiles
MapQuest also has OSM based tiles avilable, including Aerial tiles http://developer.mapquest.com/web/products/open/map