OpenStreetMap (or OSM) is a free, wiki-style, map of the world driven by user contributed content. You are able to use your own OSM tiles or ones provided through the OpenStreetMap servers. Unlike the previous third party APIs, there is no
type property to specify for this layer.
Setting up an OpenStreetMap service and tiles yourself is not too difficult, but it is outside the scope of this book (visit http://wiki.openstreetmap.org/wiki/OpenLayers_Simple_Example for more information on this). Accessing OSM with OpenLayers, however, isn't.
More information about OpenStreetMap can be found at http://www.openstreetmap.org/.