This is an old project that Andy Turner and I collaborated on sometime ago. When Google Code closed I pulled it over here as it might be useful one day.
It implements a variety of spatial cluster detection methods such as GAM, Besag & Newell, Random circles using GeoTools. There is an initial attempt at a GeoServer WPS process but I'm not sure it will still work.
If you are thinking but I have a heatmap plugin that already works you need to consider the following XKCD cartoon :
And if you feel then need for something more authorative than XKCD see what the CartoNerd has to say about heat maps. Hint he's against.