function createLabeledMarkerIcon (opts) {
  var primaryColor = opts.primaryColor || "#DA7187";
  var strokeColor = opts.strokeColor || "#000000";
  var starPrimaryColor = opts.starPrimaryColor || "#FFFF00";
  var starStrokeColor = opts.starStrokeColor || "#0000FF";
  var label = MapIconMaker.escapeUserText_(opts.label) || "";
  var labelColor = opts.labelColor || "#000000";
  var addStar = opts.addStar || false;
  
  var pinProgram = (addStar) ? "pin_star" : "pin";
  var baseUrl = "http://chart.apis.google.com/chart?cht=d&chdp=mapsapi&chl=";
  var iconUrl = baseUrl + pinProgram + "'i\\" + "'[" + label + 
      "'-2'f\\"  + "hv'a\\]" + "h\\]o\\" + 
      primaryColor.replace("#", "")  + "'fC\\" + 
      labelColor.replace("#", "")  + "'tC\\" + 
      strokeColor.replace("#", "")  + "'eC\\";
  if (addStar) {
    iconUrl += starPrimaryColor.replace("#", "") + "'1C\\" + 
        starStrokeColor.replace("#", "") + "'0C\\";
  }
  iconUrl += "Lauto'f\\";

  var icon = {}
  icon.image = iconUrl + "&ext=.png";
  return icon.image;
};


/**
* Usage -------------
*/

function createIcon(color){
  var opts = {};
  opts.primaryColor = color;
  var image = createLabeledMarkerIcon(opts);
  return image;
}

function initialize() {
  var myLatlng = new google.maps.LatLng(-25.363882,131.044922);
  var myOptions = {
    zoom: 4,
    center: myLatlng,
    mapTypeId: google.maps.MapTypeId.ROADMAP
  }
  var map = new google.maps.Map(document.getElementById("map_canvas"), myOptions);

//Create Green Icon Image
  var image = createIcon('00FF00');

//Create Marker
  var myLatLng = new google.maps.LatLng(-33.890542, 151.274856);
  var marker = new google.maps.Marker({
      position: myLatLng,
      map: map,
      icon: image
  });
}

This code was re-purposed from Pamela Fox's MapIconMaker v1.1 to work with Google Maps API v3. - new snippet as FAV toggle not working

Google Map Icon Generator

JavaScript