There are several free OSM tile servers too, so you don't even need to host yourself. MapQuest's is probably the most generous: you can use it completely free, even in paid apps and webapps. They just request that you let them know in advance if you plan to distribute an app where you anticipate very high usage (currently defined as >4,000 tiles/second). http://developer.mapquest.com/web/products/open/map