GeoCommons is a public community of GeoIQ users who are building an open repository of data and maps for the world. The GeoIQ platform includes a large number of features that empower you to easily access, visualize and analyze your data.
From About GeoCommons
So what does that mean? It means you can either upload your own data and map it or use other people's data, analyze it to your specific needs and map that. There is no software to download. It is all online.