http://www.google.com/jsapi
<script type="text/javascript">
function initialize() {
    var loc = {};
    var geocoder = new google.maps.Geocoder();
    if(google.loader.ClientLocation) {
        loc.lat = google.loader.ClientLocation.latitude;
        loc.lng = google.loader.ClientLocation.longitude;

        var latlng = new google.maps.LatLng(loc.lat, loc.lng);
        geocoder.geocode({'latLng': latlng}, function(results, status) {
            if(status == google.maps.GeocoderStatus.OK) {
                alert(results[0]['formatted_address']);
            };
        });
    }
}

google.load("maps", "3.x", {other_params: "sensor=false", callback:initialize});

Leer mas de la API de google.
https://developers.google.com/maps/articles/geolocation
https://developers.google.com/maps/documentation/javascript/examples/map-geolocation?hl=es-419&csw=1
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