Markerclusterer is a project mainly written in JavaScript, it's free.
Modification of Google Maps V3 MarkerClusterer by Luke Mahe
This is a stripped, lightweight version of MarkerClusterer by Luke Mehe that originally can be found at http://google-maps-utility-library-v3.googlecode.com/svn/trunk/markerclusterer/
This modification of the clusterer uses Google Maps original Markers to display cluster icons and therefore is missing a feature of displaying number of markers inside a cluster.