gistfile1.js

var getOSMMapType = function(){
    return new google.maps.ImageMapType({
        getTileUrl: function(coord, zoom) {
            return 'http://tile.openstreetmap.org/'+ zoom +'/'+ coord.x +'/' + coord.y +'.png';
        },
        tileSize: new google.maps.Size(256, 256),
        isPng: true,
        maxZoom: 18,
        name: "OSM"
    });
};

var mapTypes = {"OSM": getOSMMapType()};

var mapOptions = {
       "zoom": 13
              , "center": new google.maps.LatLng(someLat, someLng)
              , "mapTypeId": "OSM"
              , "mapTypeControlOptions": {
                  "mapTypeIds": mapTypes
              }
            };
var map = new google.maps.Map(document.getElementById(someMapID), mapOptions);
map.mapTypes.set("OSM", mapTypes["OSM"]);