OpenLayers point map

My favorite slippy map libraries are Mapbox and OpenLayers. Mapbox is surely the more powerful of the two and interfaces with their high quality, vector tile server. As a commercial product, though, the library is closed source and it can get expensive if you build a tool that generates a lot of interest.

The map below was built with OpenLayers and accesses the free Open Street Map tile server. The dot layer was implemented with D3. The dots represent 15 universities in the University of North Carolina system; the area of each dot is proportional to the enrollment.