Explore our planet is a nifty site. It is utilizing the RDF-based Geo-names which anyone can use (creative commons licence). Geo-names is a fabulous semantic web data-set and Explore our planet is one open way of exploring this to put attention on emerging environental issues by layering them on top of Google maps (for now).
It provides REST web access. It is good for developing web applications that require access to GIS data, geocode and similar functionality as ArcGIS.