The PELAGIOS Heatmap is a mashup of the Roman empire map by Johan Åhlfeldt and the PELAGIOS dataset. The number of geo-annotations available through the PELAGIOS network is superimposed on the map as a heatmap. There is also a search box which works through the PELAGIOS API.
Visit the online version of the mashup at http://pelagios.github.io/pelagios-heatmap.