Produced by the Center for Sustainability and the Global Environment (SAGE at the University of Wisconsin), this site brings together global GIS datasets for several environmental variables, including those of: Human Impacts (e.g. Population Growth Rate, Doubling Time), Land Use (e.g. Croplands, Suitability for Agriculture), Ecosystems (e.g. NPP, Soil Organic Carbon), Water Resources (e.g. Annual Runoff, Evapotranspiration).
Several high quality global gridded datasets are available for a variety of human impact metrics, including the alpha version of GRUMP– Global Rural-Urban Mapping Project and the Gridded Population of the World (GPW v3). The CIESIN site also serves links to other relevant datasets of US Census files and the global Last of the Wild datasets.
Still the best data resource for remotely sensed data and its products. The GLCF is a research center for land cover science, mostly remotely sensed, providing free GIS data for satellite imagery (e.g., LandSat, ASTER, SRTM, etc) and its products (e.g., AVHRR, MODIS, Forest Change, etc).