Before people get all excited: OpenStreetMap barely contains any 3D data. You can get a layout, and guess the height of the houses, if not tagged/stored in OSM, and they usually are not.
http://wiki.openstreetmap.org/wiki/Relief_maps
http://wiki.openstreetmap.org/wiki/Contours
http://www.openstreetmap.org/#map=13/33.9147/-112.4034&layer...
Then again, for common elevation data, there are loads of other good sources which can be used in conjunction with OSM, not sure about licensing though.
granted, this is probably more on the "intellectual property" and "secret sauce" side of things.