Google Maps API v3: Group markers?

Question

I'm using Google Maps with API v3. I will add many markers, now got question, is there posibility to group markers? For example, by city? I'm using a small snippet to creating sidebar with markers buttons.

Here's the code:

/**
 * map
 */
var mapOpts = {
  mapOpts: new google.maps.LatLng(60.28527,24.84864),
  mapTypeId: google.maps.MapTypeId.ROADMAP,
  scaleControl: true,
  scrollwheel: false
}
var map = new google.maps.Map(document.getElementById("map"), mapOpts);
//  We set zoom and center later by fitBounds()



/**
 * makeMarker() ver 0.2
 * creates Marker and InfoWindow on a Map() named 'map'
 * creates sidebar row in a DIV 'sidebar'
 * saves marker to markerArray and markerBounds
 * @param options object for Marker, InfoWindow and SidebarItem
 * @author Esa 2009
 */
var infoWindow = new google.maps.InfoWindow();
var markerBounds = new google.maps.LatLngBounds();
var markerArray = [];

function makeMarker(options){
  var pushPin = new google.maps.Marker({map:map});
  pushPin.setOptions(options);
  google.maps.event.addListener(pushPin, "click", function(){
    infoWindow.setOptions(options);
    infoWindow.open(map, pushPin);
    if(this.sidebarButton)this.sidebarButton.button.focus();
  });
  var idleIcon = pushPin.getIcon();
  if(options.sidebarItem){
    pushPin.sidebarButton = new SidebarItem(pushPin, options);
    pushPin.sidebarButton.addIn("sidebar");
  }
  markerBounds.extend(options.position);
  markerArray.push(pushPin);
  return pushPin;
}

google.maps.event.addListener(map, "click", function(){
  infoWindow.close();
});


/**
 * Creates a sidebar item 
 * @constructor
 * @author Esa 2009
 * @param marker
 * @param options object Supported properties: sidebarItem, sidebarItemClassName, sidebarItemWidth,
 */
function SidebarItem(marker, opts){
  var tag = opts.sidebarItemType || "button";
  var row = document.createElement(tag);
  row.innerHTML = opts.sidebarItem;
  row.className = opts.sidebarItemClassName || "sidebar_item";  
  row.style.display = "block";
  row.style.width = opts.sidebarItemWidth || "120px";
  row.onclick = function(){
    google.maps.event.trigger(marker, 'click');
  }
  row.onmouseover = function(){
    google.maps.event.trigger(marker, 'mouseover');
  }
  row.onmouseout = function(){
    google.maps.event.trigger(marker, 'mouseout');
  }
  this.button = row;
}
// adds a sidebar item to a <div>
SidebarItem.prototype.addIn = function(block){
  if(block && block.nodeType == 1)this.div = block;
  else
    this.div = document.getElementById(block)
    || document.getElementById("sidebar")
    || document.getElementsByTagName("body")[0];
  this.div.appendChild(this.button);
}
// deletes a sidebar item
SidebarItem.prototype.remove = function(){
  if(!this.div) return false;
  this.div.removeChild(this.button);
  return true;
}

/**
 * markers and info window contents
 */
makeMarker({
  position: new google.maps.LatLng(60.28527,24.84864),
  title: "Vantaankoski",
  sidebarItem: "Vantaankoski",
  content: "Vantaankoski"
});   
makeMarker({
  position: new google.maps.LatLng(60.27805,24.85281),
  title: "Martinlaakso",
  sidebarItem: "Martinlaakso",
  content: "Martinlaakso"
});  
makeMarker({
  position: new google.maps.LatLng(60.27049,24.85366),
  title: "Louhela",
  sidebarItem: "Louhela",
  content: "Louhela"
}); 
makeMarker({
  position: new google.maps.LatLng(60.26139,24.85478),
  title: "Myyrmäki",
  sidebarItem: "Myyrmäki",
  content: "Myyrmäki"
});   
makeMarker({
  position: new google.maps.LatLng(60.24929,24.86128),
  title: "Malminkartano",
  sidebarItem: "Malminkartano",
  content: "Malminkartano"
});  
makeMarker({
  position: new google.maps.LatLng(60.23963,24.87694),
  title: "Kannelmäki",
  sidebarItem: "Kannelmäki",
  content: "Kannelmäki"
}); 
makeMarker({
  position: new google.maps.LatLng(60.23031,24.88353),
  title: "Pohjois-Haaga",
  sidebarItem: "Pohjois<br>Haaga",
  content: "Pohjois-Haaga"
});   
makeMarker({
  position: new google.maps.LatLng(60.21831,24.89354),
  title: "Huopalahti",
  sidebarItem: "Huopalahti",
  content: "Huopalahti"
});   
makeMarker({
  position: new google.maps.LatLng(60.20803,24.92039),
  title: "Ilmala",
  sidebarItem: "Ilmala",
  content: "Ilmala"
});    
makeMarker({
  position: new google.maps.LatLng(60.19899,24.93269),
  title: "Pasila",
  sidebarItem: "Pasila",
  content: "Pasila"
});  
makeMarker({
  position: new google.maps.LatLng(60.17295,24.93981),
  title: "Helsinki",
  sidebarItem: "Helsinki",
  content: "Helsinki"
});    




/**
 *   fit viewport to markers
 */
map.fitBounds(markerBounds);

Answer 1

See this: http://www.appelsiini.net/2008/11/introduction-to-marker-clustering-with-google-maps

And MarkerClusterer here: http://code.google.com/p/gmaps-utility-library-dev/