You can add just about anything on OSM (http://wiki.openstreetmap.org/wiki/Category:En_key_descriptions http://wiki.openstreetmap.org/wiki/Category:En_key_descr

You can add just about anything on OSM (http://wiki.openstreetmap.org/wiki/Category:En_key_descriptions http://wiki.openstreetmap.org/wiki/Category:En_key_descripti...), so the only limit is what people actually map :) (and sometimes what the community decides, e.g. adding things that can't be seen anymore, like old tram lines, is generally frowned upon in main OSM and requested for something like Open Historical Map instead)