The latest installment of the Map for hemnet.se is now out. One of the things we added is clustering of map markers, i.e. particularly dense groups of markers are combined into a cluster marker. To reveal the contents of the cluster marker, the user zooms in a level.
The obvious benefit of this to the user is to reduce clutter. Since we are doing the clustering server-side and on the fly, there is also a significant performance gain.
Clustering algorithms is an interesting field in itself, and there are a plethora of known algorithms used for a variety of clustering applications. We ended up combining grid-based and viral growth techniques.