MarkerClusterer: A Solution to the Too Many Markers Problem, MarkerClusterer example and reference links, MarkerClusterer for Google Maps v3 reference.
Some applications are required to display a large number of locations or markers. Plotting thousands of markers on a map can quickly lead to a degraded user experience. Too many markers on the map cause both visual overload and sluggish interaction with the map. MarkerClusterer is good solution to overcome this problem.
Marker Clustering simplifies your data visualization by consolidating data that are nearby each other on the map in an aggregate form.
You can find more detail and sample code from below links
MarkerClusterer v3 Examples
Read more for MarkerClusterer from Google Developers