Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) GDEM. ASTER DEM data provides global coverage at a resolution of 90 meters. Often the highest resolution data freely available worldwide. A joint operation between NASA and Japan's Ministry of Economy, Trade and Industry, ASTER DEMs are available from a variety of sources:
Demobase gridded mapping (US Census Bureau led interagency project): 100-meter resolution gridded population datasets for select areas.
Food and Agriculture Organization of the United Nations (FAO) Aquastat: FAO's global water information system, information by country on water resources, water uses, and water managent.
Global Administrative Areas (GADM): Provides sub-national administrative boundaries, often to the third or fourth level, for every country.
Google Earth Engine: A variety of raster datasets in worldwide coverage. Includes satellite imagery (including Landsat, MODIS and Sentinel), elevation, land cover, weather, climate and population datasets.
NASA EarthData: Access to a wide variety of satellite imagery and derivative products.
National Geospatial Agency (NGA) GEOnet Name Server (GNS): the official repository of standard spellings of all foreign geographic names, sanctioned by the United States Board on Geographic Names (US BGN). The database includes variant spellings, information about location, administrative division, and quality.
Natural Earth: Public domain, small scale (1:10m, 1:50m, 1:110m) bas map features worldwide.
Open Street Map: Open source "Wiki" Map of volunteered and public domain street and feature data world-wide.
Socioeconomic Data and Applications Center (SEDAC): Wide variety of datasets focusing of human interactions in the environment. Includes Gridded Population of the World (GPW), Global Rural-Urban Mapping Project (GRUMP), Environmental Performance Index (EPI) , and Last of the Wild.
United Nations Environmental Data Explorer: Database of over 500 demographic and environmental variables at national and regional scales.
USGS Earth Explorer: Wide array of optical and radar data, weather satellite images and DEMs.
USGS Global Visualization Viewer (GLoVis): Database of satellite imagery and orthophotos. Includes Landsat, ISRO, GeoEye, Sentinel-2, EO-1 and Global Land Survey data.
Copernicus Open Access Hub: Access to Sentinel imagery and products.
WorldClim - Global Climate Data: Worldwide gridded climate data (temperature, precipitation and bioclimatic variables) up to 30 arc second resolution. Includes historical conditions and future (2050, 2070) projections.
African Marine Atlas: Broad atlas of environmental data for Africa.
European Environment Agency: Broad source of environmental data for Europe.