以下はリンク集、今後まとめる

http://openlayers.org/dev/examples/strategy-cluster-threshold.html
http://openflights.org/blog/2009/10/21/customized-openlayers-cluster-strategies/
http://openlayers.org/dev/examples/strategy-cluster-extended.html
http://openlayers.org/dev/examples/strategy-cluster.html
http://stackoverflow.com/questions/6641919/openlayers-nice-marker-clustering
http://openlayers.org/dev/examples/strategy-cluster.html