open source

You have to register but it's free data.

The have free stuff and not free stuff. All and all, lots of free data.

"Cartagen is a vector-based, client-side framework for rendering maps in native HTML 5. Written in JavaScript, it uses the new Canvas element to load mapping data from various sources, including OpenStreetMap. Maps are styled with Geographic Style Sheets (GSS), a cascading stylesheet specification for geospatial information – a decision which leverages literacy in CSS to make map styling more accessible. However, GSS is a scripting language as well, making Cartagen an ideal framework for mapping dynamic data."

An awesome GIS map application with many interactive capabilities for open source GIS.

GIS desktop service "designed for capturing, storing, handling, analyzing and deploying any kind of referenced geographic information in order to solve complex management and planning problems."

"Commonly referred to as GRASS, this is free Geographic Information System (GIS) software used for geospatial data management and analysis, image processing, graphics/maps production, spatial modeling, and visualization. GRASS is currently used in academic and commercial settings around the world, as well as by many governmental agencies and environmental consulting companies. GRASS is an official project of the Open Source Geospatial Foundation."

Also, they are apart of the Google Summer of Code.....